Add option to mirror to launchpad (for PPA)

The intention of this is to use it for the Kdenlive PPA. Since years the synchronization is done via an unreliable cron script running on one a machine of one of the Kdenlive team members, but to me it seems a good idea to move it to KDE infrastructure.

I am not finally sure if it makes sense to implement it on this level or if it would be better to do it in the CI? Or a completely different solution? To decide on that it is also important to mention that the Kdenlive PPA also provides a nightly version of mlt (Kdenlives main dependency) and that this needs mirroring too. However I wonder if we might drop this and only keep a stable mlt version in the PPA which can be mirrored by hand since the stable build needs manual actions anyway.

This is optional and will only happen if you add a line like

launchpadmirror: kdenlive

to the repo metadata, where the value is the name of the launchpad repository