Previously, users could only upload videos to third party websites and
embed them through the media dialog to add them to the website.

It is now possible for users to upload videos as ir.attachments in the
video media dialog and embed them on the website with the native HTML5
video player. The video is then stored on the local server and not on
a 3rd party website, and served directly from the same server.

This change makes it easier for users to use the website platform
without having to register to third party websites for video services.
The uploaded files are limited to the supported mp4 codecs and a file
size of 128 Mb.

Once uploaded, the video can be configured the same way an embedded
video can with options for autoplay, hiding controls and looping.
The user can access a gallery of already uploaded videos similar to
the image dialog, with thumbnails depicting the videos as well as a
preview being rendered when selecting a video or when modifying its
options.

Videos can either be embedded or selected in the video media dialog,
which is emphasized by hiding the irrelevant elements to the current
selection.

In order to upload larger files, it is necessary to also add a
client_max_body_size of at least 130M to match the internal 
size limit for video uploads (NGINX configuration).

### Technical notes:

- Background videos use the same option for autoplay and mute, since
browsers allow autoplay seemlessly only when the video is muted
(mute + defaultMute)
- Uploaded videos are added using the native HTML5 video tag in the
iframe's srcdoc. This way, any styling applied to regular embedded
videos is also applied to the uploaded videos consistently.
- Videos are limited to natively supported mp4 formats (4 differents
binary signatures) and an error is raised if the file doesn't have
one of the signatures.
- Videos are limited to 128 MB with a server-side size check raising
and error if the file is bigger.
- Thumbnails have a max height of 120px, preserve aspect ratio, and
have styling applied to center them in the attachment containers.
- Uploading and serving videos is done from 'web/content/' to
apply processing, encoding, size and format checks server-side.
- New route for asynchronously updating video thumbnails
- New placeholder video for when an attachment was deleted or removed
from the database (/web/static/video/placeholder.mp4)
- With the new changes, web_editor's ir_attachment renames image_src,
image_width and image_height to src, width and height respectively,
to accomodate for both videos and images.
- Special handling is used for fetchAttachments in order to accommodate
the case where a previously selected attachment exists, but it is not in
the first loaded attachments. An RPC is made to fetch that specific
attachment and insert it into the displayed attachments.


task-2618497
Ent PR: odoo/enterprise#22286
Upgrade PR: odoo/upgrade#3065