Add a search feature to snapshot branches view

It is now possible to filter snapshot branches according to a substring contained in their names at swh-storage level.

swh-web added support for that filtering feature by adding a branch_name_include_substring parameter to the swh.web.common.archive.lookup_snapshot function.

This calls for adding a search form in the branches view of swh-web in order to search for branches of interest.

This could be implemented by adding a new branch_name_include query parameter to the branches view and by adapting the swh.web.browse.snapshot_context.browse_snapshot_branches implementation.

---

Migrated from T3157 (view on Phabricator)