Rename event view dependencies

Seems more coherent. Added (really) small corrections too (typo,
syntax).
Add tag repository
Add tag auto-completion
Add tag deletion

Delete tags without videos
Remove datepicker border
Correct identation
Add date invalid message

Since the default date type is using a data-transformer, the entity
validation messages won’t show up.
Add poster functions to entity:Event
Add validation groups

Require a new poster file only on event creation
Add poster to event form
Add custom file input

When javascript is enabled, the standard input button is replaced by
something less ugly.
Add delete buttons to events and videos
Minor correction to event list js
Add pre-remove callbacks

Note that, for videos, I just added the structure but no actual file
deletion yet. I’ll take care of it in the add-video branch.
Add delete logic
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

I
Add a few design corrections