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.

app/services/search_indexer.rb

@@ -105,7 +105,7 @@ class SearchIndexer
   end
 
   def self.update_tags_index(tag_id, name)
-    update_index(table: 'tag', id: tag_id, raw_data: [name])
+    update_index(table: 'tag', id: tag_id, raw_data: [name.downcase])
   end
 
   def self.queue_post_reindex(topic_id)