* 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
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
