users: add stricter checks for user promotions.

New rules for user promotions:

* Moderators can no longer promote other users to moderator level. Only
  Admins can promote users to Mod level. Mods can only promote up to Builder level.
* Admins can no longer promote other users to Admin level. Only Owners
  can promote users to Admin. Admins can only promote up to Mod level.
* Admins can no longer demote themselves or other admins.

These rules are being changed to account for the new Owner user level.

Also change it so that when a user upgrades their account, the promotion
is done by DanbooruBot. This means that the inviter and the mod action
will show DanbooruBot as the promoter instead of the user themselves.

app/controllers/admin/users_controller.rb

@@ -6,7 +6,13 @@ module Admin
 
     def update
       @user = authorize User.find(params[:id]), :promote?
-      @user.promote_to!(params[:user][:level], params[:user])
+
+      @level = params.dig(:user, :level)
+      @can_upload_free = params.dig(:user, :can_upload_free)
+      @can_approve_posts = params.dig(:user, :can_approve_posts)
+
+      @user.promote_to!(@level, CurrentUser.user, can_upload_free: @can_upload_free, can_approve_posts: @can_approve_posts)
+
       redirect_to edit_admin_user_path(@user), :notice => "User updated"
     end
   end

app/controllers/user_upgrades_controller.rb

@@ -48,7 +48,7 @@ class UserUpgradesController < ApplicationController
         :card => params[:stripeToken],
         :description => params[:desc]
       )
-      @user.promote_to!(level, is_upgrade: true)
+      @user.promote_to!(level, User.system, is_upgrade: true)
       flash[:success] = true
     rescue Stripe::CardError => e
       DanbooruLogger.log(e)

app/logical/user_promotion.rb

@@ -1,33 +1,28 @@
 class UserPromotion
-  attr_reader :user, :promoter, :new_level, :options, :old_can_approve_posts, :old_can_upload_free
+  attr_reader :user, :promoter, :new_level, :old_can_approve_posts, :old_can_upload_free, :can_upload_free, :can_approve_posts, :is_upgrade
 
-  def initialize(user, promoter, new_level, options = {})
+  def initialize(user, promoter, new_level, can_upload_free: nil, can_approve_posts: nil, is_upgrade: false)
     @user = user
     @promoter = promoter
-    @new_level = new_level
-    @options = options
+    @new_level = new_level.to_i
+    @can_upload_free = can_upload_free
+    @can_approve_posts = can_approve_posts
+    @is_upgrade = is_upgrade
   end
 
   def promote!
-    validate
+    validate!
 
     @old_can_approve_posts = user.can_approve_posts?
     @old_can_upload_free = user.can_upload_free?
 
     user.level = new_level
+    user.can_upload_free = can_upload_free unless can_upload_free.nil?
+    user.can_approve_posts = can_approve_posts unless can_approve_posts.nil?
+    user.inviter = promoter
 
-    if options.key?(:can_approve_posts)
-      user.can_approve_posts = options[:can_approve_posts]
-    end
-
-    if options.key?(:can_upload_free)
-      user.can_upload_free = options[:can_upload_free]
-    end
-
-    user.inviter_id = promoter.id
-
-    create_user_feedback unless options[:is_upgrade]
-    create_dmail unless options[:skip_dmail]
+    create_user_feedback unless is_upgrade
+    create_dmail
     create_mod_actions
 
     user.save
@@ -45,20 +40,21 @@ class UserPromotion
     end
 
     if user.level_changed?
-      category = options[:is_upgrade] ? :user_account_upgrade : :user_level_change
+      category = is_upgrade ? :user_account_upgrade : :user_level_change
       ModAction.log(%{"#{user.name}":/users/#{user.id} level changed #{user.level_string_was} -> #{user.level_string}}, category)
     end
   end
 
-  def validate
-    # admins can do anything
-    return if promoter.is_admin?
-
-    # can't promote/demote moderators
-    raise User::PrivilegeError if user.is_moderator?
-
-    # can't promote to admin
-    raise User::PrivilegeError if new_level.to_i >= User::Levels::ADMIN
+  def validate!
+    if !promoter.is_moderator?
+      raise User::PrivilegeError, "You can't promote or demote other users"
+    elsif promoter == user
+      raise User::PrivilegeError, "You can't promote or demote yourself"
+    elsif new_level >= promoter.level
+      raise User::PrivilegeError, "You can't promote other users to your rank or above"
+    elsif user.level >= promoter.level
+      raise User::PrivilegeError, "You can't promote or demote other users at your rank or above"
+    end
   end
 
   def build_messages

app/models/user.rb

@@ -250,8 +250,8 @@ class User < ApplicationRecord
       end
     end
 
-    def promote_to!(new_level, options = {})
-      UserPromotion.new(self, CurrentUser.user, new_level, options).promote!
+    def promote_to!(new_level, promoter = CurrentUser.user, **options)
+      UserPromotion.new(self, promoter, new_level, **options).promote!
     end
 
     def promote_to_admin_if_first_user