Update timezone-field and add pytz to requirements, add matching migration

Previously, the version of pytz was not fixed. This could lead to problems when the list of timezones of the locally installed version of pytz differed from the list fixed in migration 0027 of AKModel. This commit therefore:
* updates django-timezone-field to the newest version
* Adds pytz with the current version to requirements.txt
* Introduces a migration to reflect the changes in the internal timezone list of pytz (--('America/Godthab', 'GMT-03:00 America/Godthab'), ++('America/Nuuk', 'GMT-03:00 America/Nuuk'), --('Australia/Currie', 'GMT+11:00 Australia/Currie'))