zulip: user status: Add timeout, and other next steps to modal.

Putting some of the reasoning here, so it doesn’t get lost. But some of the goals of the status feature are:

• Make it easy to set a status or mark yourself as unavailable for N minutes
• Make it hard (but possible) to set a status forever

For that, the user status modal should look like the following:

**Set a status**

[ ] Set yourself as unavailable (unavailable-icon)
[ ] Set status to [text                 ]

Clear after [30 minutes]

[Cancel] [Save]

The options for the dropdown can be:

• 30 minutes
• 1 hour
• 4 hours
• Today
• This week
• Custom date and time

Other related things:

• We should also change the modal to use our regular modal styling
• In the user popover, “Set yourself as unavailable” should have a submenu that allows “for 30 minutes”, “for 1 hour”, etc.

Not sure the right way to handle this on the backend, but possibly it would make sense to just have a ScheduledFunction table that gives a way to schedule arbitrary function calls in the future?