Add a menu item to delete both local and remote branch at once

[stefanhaller]

• PR Description

Add a menu item to delete both local and remote branch at once.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.01%	✅ 91.67%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (ba7d2f0)	50423	43306	85.89%
Head commit (3cd47f3)	50518 (+95)	43391 (+85)	85.89% (+0.01%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3916)	96	88	91.67%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[stefanhaller]

I decided to put up only a single confirmation ("are you sure you want to delete both branches"). If the local branch is not merged, I append the warning about this below in the same popup. I like this better than putting up a separate popup for this after the first one, but I can change it if you disagree.