Adding 1 hour expiration on seat map caching.

Because of no discrete time steps within the system, a transaction can go from
pending to expired at any given time. Its status is calculated at the time it
is needed. This resulted in invalid seat maps when users of pending tickets
had started to reserve their seats and failed to pay for the tickets.
Non-relative paths in transactions#show.

Since the front page and event pages are not on the same subdomain, paths in
transactions#show can't be relative. This fixes that.
Adding helper method for md parsing translations.

This helps removing line wrapped string interpolations within markdown parsed
blocks in HAML, which my editor really complained about.
Updating the TODO with a couple of items.