Add ability for an administrator to remove any video (#61)

Add ability for an administrator to remove any video (#61)

* Add ability for an admin to remove every video on the pod.

* Server: add BlacklistedVideos relation.

* Server: Insert in BlacklistedVideos relation upon deletion of a video.

* Server: Modify BlacklistedVideos schema to add Pod id information.

* Server: Moving insertion of a blacklisted video from the `afterDestroy` hook into the process of deletion of a video.

To avoid inserting a video when it is removed on its origin pod.
When a video is removed on its origin pod, the `afterDestroy` hook is fire, but no request is made on the delete('/:videoId') interface.
Hence, we insert into `BlacklistedVideos` only on request on delete('/:videoId') (if requirements for insertion are met).

* Server: Add removeVideoFromBlacklist hook on deletion of a video.

We are going to proceed in another way :).
We will add a new route : /:videoId/blacklist to blacklist a video.
We do not blacklist a video upon its deletion now (to distinguish a video blacklist from a regular video delete)
When we blacklist a video, the video remains in the DB, so we don't have any concern about its update. It just doesn't appear in the video list.
When we remove a video, we then have to remove it from the blacklist too.
We could also remove a video from the blacklist to 'unremove' it and make it appear again in the video list (will be another feature).

* Server: Add handler for new route post(/:videoId/blacklist)

* Client: Add isBlacklistable method

* Client: Update isRemovableBy method.

* Client: Move 'Delete video' feature from the video-list to the video-watch module.

* Server: Exclude blacklisted videos from the video list

* Server: Use findAll() in BlacklistedVideos.list() method

* Server: Fix addVideoToBlacklist function.

* Client: Add blacklist feature.

* Server: Use JavaScript Standard Style.

* Server: In checkUserCanDeleteVideo, move the callback call inside the db callback function

* Server: Modify BlacklistVideo relation

* Server: Modifiy Videos methods.

* Server: Add checkVideoIsBlacklistable method

* Server: Rewrite addVideoToBlacklist method

* Server: Fix checkVideoIsBlacklistable method

* Server: Add return to addVideoToBlacklist method