It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
* 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
