Set user availability [BE]

Problem to solve

This is the backend implementation issue for the feature to set user availability. Specifically, the backend must be able to support the "busy" checkbox that will be added to the user status modal.

Proposal

• User should have an "availability" enum attribute. (While there is currently only 1 availability option of "busy", there are strong plans to add more options like "OOO", so we probably should NOT implement "busy" as a boolean attribute.)
• This attribute should be exposed for the following features:
  • User status modal (accessible from the "edit profile" page)
  • Profile page
  • user_menu in the top right of the navbar
  • user details hover at the top of issuables and notes
  • user details hover on username autocomplete (when typing in notes)

Related links

• This "availability" feature is needed to un-block the Merge Request reviewer workload dashboard.