Edit pages

This pull request adds pages and logic to add, edit and delete videos and events (resolves #24) 

It also corrects multiple bugs, and resolves #52.

Missing features:
- Video files deletion is yet to be implemented. It depends on how the upload is handled, and will therefore be part of the next PR (add-video)
- Same thing about redirections from video edit pages (they should point to the video_add page, which doesn't exist yet)