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.

config/locales/server.en.yml

@@ -1798,6 +1798,7 @@ en:
     min_trust_level_to_tag_topics: "Minimum trust level required to tag topics"
     suppress_overlapping_tags_in_list: "If tags match exact words in topic titles, don't show the tag"
     remove_muted_tags_from_latest: "Don't show topics tagged with muted tags in the latest topic list."
+    force_lowercase_tags: "Force all new tags to be entirely lowercase."
 
     company_short_name: "Company Name (short)"
     company_full_name: "Company Name (full)"

config/site_settings.yml

@@ -1788,3 +1788,5 @@ tags:
     client: true
   remove_muted_tags_from_latest:
     default: false
+  force_lowercase_tags:
+    default: true