Rebuild by setting a tag

[ligi]

When I set the tag "rebuild" for a commit - continuum should trigger the rebuild and remove the tag

this is e.g. needed when the UI-tests fail for some real device fuckup - currently I am rebasing to get a new commit-hash and trigger a rebuild this way - but the tag is a way more elegant solution.

could also imagine tags for other 'special tasks' like fullbuild (no singleFlavor) or generating screenshots for different languages,