FEATURE: Add timezone to core user_options (#8380)

* Add timezone to user_options table

* Also migrate existing timezone values from UserCustomField,
  which is where the discourse-calendar plugin is storing them

* Allow user to change their core timezone from Profile

* Auto guess & set timezone on login & invite accept & signup

* Serialize user_options.timezone for group members. this is so discourse-group-timezones can access the core user timezone, as it is being removed in discourse-calendar.

* Annotate user_option with timezone

* Validate timezone values

app/controllers/groups_controller.rb

@@ -269,7 +269,7 @@ class GroupsController < ApplicationController
       end
     end
 
-    users = users.select('users.*, group_users.created_at as added_at')
+    users = users.joins(:user_option).select('users.*, user_options.timezone, group_users.created_at as added_at')
 
     members = users
       .order('NOT group_users.owner')