Previously swh-web was only supporting the browsing of snapshot branches
targeting revisions but we recently introduced new Software Heritage
loaders producing snapshots whose branches target contents or directories
but no revisions.
So add support to browse such snapshots as previously they were flagged as
empty when trying to display them.
Related to T4669
This is the first part for properly supporting the browsing of such snapshots
The second diff on the subject will update the view for browsing the branches
list of a snapshot by adding the target type of each branch (as in the releases
list view) and generating proper links to browse those targetting contents or