Add field to store the timezone of a meeting
Remove code that is not used anywhere but in the unit-tests
Fix converting the meeting from their own timezone to the user's one 

And propagate the timezone information to the backend when editing/adding a meeting.
Add alembic revision to add the meeting_timezone field in the meetings table

- Tested on pg
Merge pull request #70 from fedora-infra/add_tz_to_db

Add tz to db