Refs view: branch deletion

[actionshrimp]

Hi there! A small QoL improvement for the refs view from magit. I suspect I may have missed an edge case or two, but it is still pretty handy for bulk-deletion when you have lots of old branches hanging round.

• Allows deleting a branch from the refs view more easily with a direct bind ('x' by default), similar to magit.
• Allows deleting multiple branches simultaneously with a visual selection which makes cleaning up lots of branches much simpler.

I wasn't entirely sure if making a new sub-section refs_view in the mappings was the ideal option as it didn't exist already and most of the other mappings it uses open other popups, but that's what I went for - happy to amend if it makes sense to do it a different way.

refs-view-branch-delete.mp4

[actionshrimp]

Not completely sure why I'm getting that linter failure as I didn't touch that file, but it seems ok locally and on master.

[CKolkey]

this is dope as hell, thanks for being patient :)