browse: Filter out pull request branches in snapshot content
Github origins might contain a lot of branches corresponding to pull
requests in their associated snapshots.
Those branches are not really of interest and add a lot of noise in
the branches views of swh-web.
So filter out these branches from snapshots by default in the browse
web application in order to improve the display.
Nevertheless, those branches are not filtered out when querying a
snapshot with the Web API.
Related to T2782