* 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
`Upload#url` is more likely and can change from time to time. When it
does changes, we don't want to have to look through multiple tables to
ensure that the URLs are all up to date. Instead, we simply associate
uploads properly to `UserProfile` so that it does not have to replicate
the URLs in the table.
