forum: remove ability to merge forum topics.

* Rarely used (only used ~15 times in total, not used at all since 2015-2016).
* Merging topics didn't properly bump the new topic.
* Merging topics didn't log a modaction when the old topic was deleted.
* Merging topics broke the old topic. Moving all the posts from one topic
  to another leaves the old topic with zero posts. This normally can't
  happen and it causes exceptions when you try to view the empty topic.
* It was technically possible to merge a topic with itself. This would
  break the response_count.
* It was technically possible for a mod to merge a topic into an
  admin-only topic.

app/controllers/forum_topics_controller.rb

@@ -2,10 +2,9 @@ class ForumTopicsController < ApplicationController
   respond_to :html, :xml, :json
   respond_to :atom, only: [:index, :show]
   before_action :member_only, :except => [:index, :show]
-  before_action :moderator_only, :only => [:new_merge, :create_merge]
   before_action :normalize_search, :only => :index
-  before_action :load_topic, :only => [:edit, :show, :update, :destroy, :undelete, :new_merge, :create_merge]
-  before_action :check_min_level, :only => [:show, :edit, :update, :new_merge, :create_merge, :destroy, :undelete]
+  before_action :load_topic, :only => [:edit, :show, :update, :destroy, :undelete]
+  before_action :check_min_level, :only => [:show, :edit, :update, :destroy, :undelete]
   skip_before_action :api_check
 
   def new
@@ -79,15 +78,6 @@ class ForumTopicsController < ApplicationController
     redirect_to forum_topics_path, :notice => "All topics marked as read"
   end
 
-  def new_merge
-  end
-
-  def create_merge
-    @merged_topic = ForumTopic.find(params[:merged_id])
-    @forum_topic.merge(@merged_topic)
-    redirect_to forum_topic_path(@merged_topic)
-  end
-
   private
 
   def normalize_search

app/models/forum_topic.rb

@@ -157,12 +157,6 @@ class ForumTopic < ApplicationRecord
     (response_count / Danbooru.config.posts_per_page.to_f).ceil
   end
 
-  def merge(topic)
-    ForumPost.where(:id => self.posts.map(&:id)).update_all(:topic_id => topic.id)
-    topic.update(response_count: topic.response_count + self.posts.length, updater_id: CurrentUser.id)
-    self.update_columns(:response_count => 0, :is_deleted => true, :updater_id => CurrentUser.id)
-  end
-
   def delete!
     update(is_deleted: true)
   end

app/views/forum_topics/_secondary_links.html.erb

@@ -36,7 +36,6 @@
         <% else %>
           <%= subnav_link_to "Sticky", forum_topic_path(@forum_topic, :forum_topic => {:is_sticky => true}), :method => :put, :data => {:confirm => "Are you sure you want to sticky this forum topic?"} %>
         <% end %>
-        <%= subnav_link_to "Merge", new_merge_forum_topic_path(@forum_topic) %>
       <% end %>
     <% end %>
   <% end %>

app/views/forum_topics/new_merge.html.erb

@@ -1,23 +0,0 @@
-<div id="c-forum-topics">
-  <div id="a-merge">
-    <h1>Merge Forum Topic</h1>
-
-    <p>Merge <strong><%= @forum_topic.title %></strong> into:</p>
-
-    <%= form_tag(create_merge_forum_topic_path(@forum_topic)) do %>
-      <div>
-        <label>
-          Forum Topic ID:
-          <%= text_field_tag :merged_id %>
-        </label>
-      </div>
-      <%= submit_tag "Merge" %>
-    <% end %>
-  </div>
-</div>
-
-<%= render "secondary_links" %>
-
-<% content_for(:page_title) do %>
-  Merge Forum Topic - <%= Danbooru.config.app_name %>
-<% end %>

config/routes.rb

@@ -140,8 +140,6 @@ Rails.application.routes.draw do
   resources :forum_topics do
     member do
       post :undelete
-      get :new_merge
-      post :create_merge
     end
     collection do
       post :mark_all_as_read

test/unit/forum_topic_test.rb

@@ -111,19 +111,6 @@ class ForumTopicTest < ActiveSupport::TestCase
       end
     end
 
-    context "#merge" do
-      setup do
-        @topic2 = create(:forum_topic, title: "yyy", creator: @user)
-        FactoryBot.create(:forum_post, :topic_id => @topic.id, :body => "xxx")
-        FactoryBot.create(:forum_post, :topic_id => @topic2.id, :body => "xxx")
-      end
-
-      should "merge all the posts in one topic into the other" do
-        @topic.merge(@topic2)
-        assert_equal(2, @topic2.posts.count)
-      end
-    end
-
     context "constructed with nested attributes for its original post" do
       should "create a matching forum post" do
         assert_difference(["ForumTopic.count", "ForumPost.count"], 1) do