add a "Filter Pull Requests" checkbox (or similar) in the Branches view of an origin in the web UI

Currently, looking for a tag using the Web UI for an active github project is very annoying because pull requests are stored as branches. For example, when a user ask question on the release vs. tag on the DGtal repository, it's almost impossible to find the tags stored in SWH for this repository because one has to click the "next" button a tremendous number of times before reaching the /ref/tag/ "section" (without any obvious way of "fast-forwarding")

Note that this is not the same as swh-loader-git#2459, which copes with this problem from loader point of view. But whenever this later task is done, we still have existing snapshots with gazillions of "PR branches".

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Migrated from T2782 (view on Phabricator)