v0.1.21: All message-crunching through cacheable Jinja2 templatess

Move postutils code into templating for the
purpose of making all message-parsing go through
Jinja2. This is resource-intensive, but the
solution was implementing Flask-Caching for list
& thread views...

Makes a very simple implementation of caching.
An improvement would be to make it so specific
thread view caches could be invalidated if new
post is a reply to said specific thread, also probably
invalidating searches, to.