TPAC - Add parse_datetime to expire_date

As reported by paxed in IRC, sometimes the function for expire_date has
troubles and you get an apache internal server error page and log errors
like:

egweb: template error: date error - bad time/date string:
expects 'h:m:s d:m:y'  got: '2015-01-31T00:00:00+0200'

Adding the ctx.parse_datetime() function to wrap the ctx.user.expire_date
resolves this problem.

Signed-off-by: Ben Shum <bshum@biblio.org>
Signed-off-by: Pasi Kallinen <pasi.kallinen@pttk.fi>