Add JSON API support for timezone update (#2806)

* Add JSON API support for timezone update

* Simplify timezone controller to just head :no_content

* Use PUT instead of PATCH in timezone tests to match docs

* Use PATCH instead of PUT for timezone endpoint

Reverts tests to use patch and updates docs heading to PATCH
for consistency.

app/controllers/my/timezones_controller.rb

@@ -1,6 +1,7 @@
 class My::TimezonesController < ApplicationController
   def update
     Current.user.settings.update!(timezone_name: timezone_param)
+    head :no_content
   end
 
   private