Add time preferences to user [BE]

Problem to solve

We'd like to make displayed times in GitLab more customizable:

• Set timezone
• Change timezone to 24h
• Absolute times vs. relative

In order to enable these changes, we need to support these settings on the backend.

We're looking at these changes on the frontend in https://gitlab.com/gitlab-org/gitlab-ce/issues/56997.

Proposal

• Add a user attribute to set a user's time zone.
  • See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/999 for how we'd like to add this: a field changed in /profile via a dropdown. Format: (GMT -08:00) Pacific Time (US & Canada). Default to UTC.
• Add user attributes for time display preferences:
  • Changing how times are displayed throughout the UI: 12-hour format or 24-hour format. Default to 12-hour format.
  • Changing whether or not relative times are displayed: relative or absolute.
    • Relative is "two weeks ago" vs. absolute "Sept 7th, 2019 5:00pm". Default to relative times.

We should be able to set the timezone attribute via the Users API. 12/24h and absolute/relative should be specific to the user's preferences and shouldn't be modifiable via the API (since they only apply to the UI).

We'll also probably need a migration to update this/set defaults for existing users.