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.

spec/models/tag_spec.rb

@@ -27,6 +27,12 @@ describe Tag do
       expect(event[:event_name]).to eq(:tag_created)
       expect(event[:params].first).to eq(subject)
     end
+
+    it 'prevents case-insensitive duplicates' do
+      Fabricate.build(:tag, name: "hello").save!
+      expect { Fabricate.build(:tag, name: "hElLo").save! }.to raise_error(ActiveRecord::RecordInvalid)
+    end
+
   end
 
   describe 'destroy' do