This adds a site setting (default off) to optionally show a user's local time and timezone in their user card. For example, I live in Brisbane, and if at 3:30PM my time I were to open a user who lives in California's card I would see 22:30 (PST).
* PERF: Allow passing an existing list of user field ids when loading
This avoids the need for running `UserField.pluck(:id)` for each user that is serialized
* Memoize user_fields to avoid rebuilding hash ever time
Adds a new route `/u/{username}/card.json`, which has a reduced number of fields. This change is behind a hidden site setting, so we can test compatibility before rolling out.
