Jump to pinned message keyboard shortcut

[edemaine]

Proposal: Typing p3 (maybe followed by enter?) jumps to pinned message number 3.

This gets especially interesting when you're just displaying a subthread that doesn't contain pinned message 3. Then I think we should reroot to pinned message 3. (Is this how the pinned message links work? It should be.) This makes it nicer to jump between subthreads, without ever rendering the whole thread. [Yev]