FEATURE: Mixed case tagging (#6454)

- By default, behaviour is not changed: tags are made lowercase upon creation and edit.

- If force_lowercase_tags is disabled, then mixed case tags are allowed.

- Tags must remain case-insensitively unique. This is enforced by ActiveRecord and Postgres.

- A migration is added to provide a `UNIQUE` index on `lower(name)`. Migration includes a safety to correct any current tags that do not meet the criteria.

- A `where_name` scope is added to `models/tag.rb`, to allow easy case-insensitive lookups. This is used instead of `Tag.where(name: "blah")`.

- URLs remain lowercase. Mixed case URLs are functional, but have the lowercase equivalent as the canonical.

lib/topic_query.rb

@@ -634,11 +634,12 @@ class TopicQuery
       result = result.preload(:tags)
 
       if @options[:tags] && @options[:tags].size > 0
+        @options[:tags].each { |t| t.downcase! if t.is_a? String }
 
         if @options[:match_all_tags]
           # ALL of the given tags:
           tags_count = @options[:tags].length
-          @options[:tags] = Tag.where(name: @options[:tags]).pluck(:id) unless @options[:tags][0].is_a?(Integer)
+          @options[:tags] = Tag.where_name(@options[:tags]).pluck(:id) unless @options[:tags][0].is_a?(Integer)
 
           if tags_count == @options[:tags].length
             @options[:tags].each_with_index do |tag, index|
@@ -654,7 +655,7 @@ class TopicQuery
           if @options[:tags][0].is_a?(Integer)
             result = result.where("tags.id in (?)", @options[:tags])
           else
-            result = result.where("tags.name in (?)", @options[:tags])
+            result = result.where("lower(tags.name) in (?)", @options[:tags])
           end
         end
       elsif @options[:no_tags]