Summary:
I will use it for highlighting users which are not currently available.
Maybe I will also use it in the nagging tool.
I don't plan creating a UI for it as API is currently enough for us.
Maybe I will visualize it at /calendar/ later.
I plan creating `user.deletestatus` method when this one will be done.
Test Plan:
`storage upgrade`
Call Conduit `user.addstatus`.
Verify DB.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Koolvin
Differential Revision: https://secure.phabricator.com/D2382
