users: move account deletion endpoint to /users/:id/deactivate.

Move the account deletion endpoint from /maintenance/users/deletion to either:

* https://danbooru.donmai.us/users/deactivate
* https://danbooru.donmai.us/users/:id/deactivate

This incidentally allows the Owner-level user to deactivate accounts belonging to other users. This
is meant for things like deactivating inactive accounts with invalid or abusive names. This is
limited to accounts below Gold level for security.

app/controllers/users_controller.rb

@@ -114,6 +114,32 @@ class UsersController < ApplicationController
     end
   end
 
+  def deactivate
+    if params[:id].present?
+      @user = authorize User.find(params[:id])
+    else
+      @user = authorize CurrentUser.user
+    end
+
+    respond_with(@user)
+  end
+
+  def destroy
+    @user = authorize User.find(params[:id])
+
+    user_deletion = UserDeletion.new(user: @user, deleter: CurrentUser.user, password: params.dig(:user, :password), request: request)
+    user_deletion.delete!
+
+    if user_deletion.errors.none?
+      session.delete(:user_id)
+      flash[:notice] = "Your account has been deactivated"
+      respond_with(user_deletion, location: posts_path)
+    else
+      flash[:notice] = user_deletion.errors.full_messages.join("; ")
+      redirect_to deactivate_user_path(@user)
+    end
+  end
+
   def custom_style
     @custom_css = CurrentUser.user.custom_css
     expires_in 10.years