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.

script/bulk_import/discourse_merger.rb

@@ -271,7 +271,7 @@ class BulkImport::DiscourseMerger < BulkImport::Base
     @raw_connection.copy_data(sql, @encoder) do
       source_raw_connection.exec("SELECT #{columns.map { |c| "\"#{c}\"" }.join(', ')} FROM tags").each do |row|
 
-        if existing = Tag.where(name: row['name']).first
+        if existing = Tag.where_name(row['name']).first
           @tags[row['id']] = existing.id
           next
         end