Add delete logic
Amends 5c54482
Add delete modal form

I use a form instead of a simple link to make sure that cannot reach
the delete page without actually clicking a delete button. Moreover, it
makes it easier to add more complex confirmation if we feel like it
(like asking people to enter the name of the event as confirmation)
Add modal js integration and disabled-js pages
Add a few design corrections
Enable modal on video page

Minor changes to modals, coments.
Enable session flashes

Mopa bootstrap session flashes are based on Symfony2 session flashbags
and Bootstrap alerts. Session flashes are available on every page of
the website extending base-header-navbar, but the twig extension will
be loaded only if there is an alert to display.
Add session flashes on edit pages
Override mopa template

This file is an exact copy of the corresponding MopaBootstrap file, but
for 2 lines (identified with {# ZBOUBIDOO #} tags). This solves
form-error-messages display issues (for date type and file-input type).
Note that those problems are solved in the current release of Bootstrap
(which we cannot use yet because leafo isn’t working with the very last
release of bootstrap less).
Resolve bug in event list
Add a condition to prevent a template bug when there are no events.