install: Add --reinstall

If you're installing something and its already installed, we undeploy
the old install first before deploying the new. This makes it very
easy to switch an application from one remote to another, without
having to uninstall first, which is both painful and could cause
the download to be unnecessary large.

Closes: #1241
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f