enable landing patches via the web UI for all repos

As shown in swh-docs!34 (closed) our processes for landing patches coming from external contributors aren't great (as in: not terribly user friendly).

I //think// historically asking contributors to just "git push" once their diffs are accepted used to work, but even if it is the case that isn't a great workflow, so I don't think we should try to fix it that way. Rather, we should make it trivial for us, core developers, to land accepted patches.

Again, I //think// some repos allow landing via the Phabricator web UI, e.g., swh-web, but not all of them to.

We should fix this, making sure all repos allows via-the-web landing and make sure all repos created in the future allow that too.

cc: @olasd as I think he is the person who made the magic possible for swh-web.

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Migrated from T1689 (view on Phabricator)