fix date line being off by 1 message - #552

fixes #536

consider the following three messages:

• [sent on day 1] message A
• [sent on day 1] message B
• [sent on day 2] message C

the previous code iterated messages in reverse order (i.e. C, then B, then A). Upon encountering message B, previous_msg_day is day 2, from message C - so display_date_line would trigger and return Some("=== day 2 ===") (the day of the previous_msg_day). display_message would then put === day 2 === at the TOP of message B (not the bottom!). This caused message B to be in the "wrong day".

There's two ways to fix this: put === day 2 === at the bottom of message B, or, detect that we need a date line while processing message C (not B), and continue putting the date line at the top of the message. This PR does the latter. This also has the (intended) side effect of the first message ever sent in a channel now has a date line, it previously did not.

thanks for making/working on gurk, it's so hecking nice to be able to use signal on a desktop without using electron! ❤️