When thrown into Read Mentions, show the new mention.

When thrown into Read Mentions, show the new mention.

This adds an 'is_interrupt' flag to every change of activity; the only
activity constructor that uses it is mentions(), which uses it to vary
the policy about file positioning. When you enter Read Mentions on
your own initiative, you get the same position you were at before, but
when _thrown_ into it, you go to the new message.

In order to still show the new message when you're _already_ in your
mentions, we remove ReadMentions from the list of activities that
don't cause a reload of ReadMentions.