Add commitment renewal endpoint

[VoigtS]

This endpoint aims to make commitments renewable if they are within the allowed renewal timeframe.

Current issue is that the commitment that was renewed owns a was_extended state, but does not carry the successor ID yet. That begs the question if the state itself should contain it or if it should be handled by the superseded context.
That on the other hand would require to either set the commitment as superseded when the commitment is ultimately marked as expired or if just the superseded Context is considered in this circumstance.

I change this to a regular PR now due to (maybe upcoming urgency about this topic?), it has to be noted that the UI expects was_extended from the get commitments API in order to not list already renewed commitments.

TODO's:

• I updated the documentation to describe the semantical or interface changes I introduced.

[coveralls]

coverage: 79.457% (+0.03%) from 79.426%
when pulling 3d0d11f on renew
into 94a2002 on master.

[VoigtS]

UI draft: sapcc/LimesUI#36
API declarations: sapcc/go-api-declarations#43