[imp] timoutReminder JS in templates/compose/javascript.inc

[liamr@umich.edu]

I'm really happy to see this function in there considering some of the stuff
we've run into.

I'm curious, how was 5 minutes decided upon.. because it's safe guess, or is
there a server setting or browser quirk you're taking into consideration?

I really like the reminder, but it's annoying in its frequency, and in that your
cursor returns to the top of the composition textarea when the window refreshes.

We found by upping session.gc_maxlifetime and session.cache_expire that we were
able to increase the amount of time you could take composing a message.  I've
been able to start composition, walk away, finish and send it two hours later.

Liam