Fix edition of events with files

This patch fixes a bug where it was not possible to edit an event which contained an uploaded image without re-uploading the image, as the file validators would fail to run correctly. This was due to two things:

• First, our custom Storage did not implement a size() method, preventing the validator to run on already uploaded files, and
• Second, our validator for file formats was buggy. Replaced it with Django's builtin one, which seems to not suffer the same issue.

Fixes #54 (closed)