CFP and other conference time checks should use local time, not UTC

[malemburg]

We have always assumed that the times used to check against the dates we enter on the conference models refers to local time (CET or CEST). This was also the assumption for the start of the CFP on April 26 00:00:00. However, the system had not opened the CFP yet.

Looking at the code, this appears to be a wrong assumption. Instead, UTC is used as basis,

Since we always use CET/CEST for times reported as deadlines on the website, we should fix this to use local time instead.

[malemburg]

The fix should be simple: replace timezone.now() with timezone.localtime() where those checks are done in the code (e.g. on the Conference model).