Use more limited set of timezones

In the UI, we'd like to use the list of timezones supported by Twitter (https://twitter.com/settings/account), which I believe is the same as the list in `oae-util/timezones-rails.json`. This is mostly because the full list of timezones we support is too long and too inconsistent for end users.

I think this means that we need to map timezones that are not in this timezone back to this more limited list. It might also make sense to create an endpoint that returns the list of available (Twitter) timezones, their readable name and their current GMT offset. This can then be used by the back-end and would ensure closer co-ordination between the 2.

Assigning to @stuartf