This commit adds an API endpoint for notifying the preview server about PR
updates (`/pr-updated`). According to the update, the preview server can take
several actions. Currently, it will only check and (if necessary) update the
PR's preview visibility (but more actions could be supported in the future).
The API can be used with an automatic trigger (e.g. a GitHub webhook) to
instantly update a PR's preview visibility when it changes.
Fixes#16526
Use the 7 first characters of the 40-chars long SHAs for shorter/cleaner URLs.
The collision probability is extremely low (since all SHAs are further
"namespaced" under the corresponding PR). In case of a collision, the second PR
will not be deployed, in order to avoid overwriting the original build.
(This is a design decision to keep the implementation simple. It can be changed
later if necessary.)
