Announcements timer enhancements

[robere2]

There are a number of nice-to-have additions to announcements which should all be relatively simple:

• Default timer settings
  • I.e., what's the default behavior for what happens to an announcement when it's timer ends?
  • These should probably persist across reload, but not between users (i.e., store in local storage)
• Remove a timer from an announcement
  • This can currently be accomplished by just setting the time to 0 and telling it to remove the timer, but that's not very clean.
• Add a timer to an announcement after it's already been created