Fix possible inconsistencies in tag search (#14906)

Do not downcase the queried tag before passing it to postgres when searching:
- tags are not downcased on creation
- `arel_table[:name].lower.matches(pattern)` generates an ILIKE anyway
- if Postgres and Rails happen to use different case-folding rules,
  downcasing before query but not before insertion may mean that some
  tags with some casings are not searchable

app/models/tag.rb

@@ -126,7 +126,7 @@ class Tag < ApplicationRecord
     end
 
     def search_for(term, limit = 5, offset = 0, options = {})
-      normalized_term = normalize(term.strip).mb_chars.downcase.to_s
+      normalized_term = normalize(term.strip)
       pattern         = sanitize_sql_like(normalized_term) + '%'
       query           = Tag.listable.where(arel_table[:name].lower.matches(pattern))
       query           = query.where(arel_table[:name].lower.eq(normalized_term).or(arel_table[:reviewed_at].not_eq(nil))) if options[:exclude_unreviewed]