Trigger CI with a PR comment

[matthiasgoergens]

Sometimes our CI fails for reasons unrelated to the Pull Request.

I usually retrigger it by push an empty commit created via something like git commit --allow-empty -m "Tickle CI".

It would be useful to allow people to just make a comment on the PR like this

Retrigger CI

instead of having to push a pro forma commit.

You can use the docs for some inspiration.

[matthiasgoergens]

I specifically wanted this for eg #1689