key configuration

[kohlhase]

We need a key pair for each mathhub docker, so that the worker can commit to gl.mathhub.info. The current solution in lmh, i.e. to copy the user's key pair into the lmh docker is not acceptable. We need a key configuration dialog in MathHub that guides the new user through this.

1. generate a ssh key
2. prompt the user to copy it to https://gl.kwarc.info or better use the GitLab API for doing this directly, so that the user only needs to authenticate.

[tkw1536]

We could mount a directory from the real system with existing ssh keys in the right place (this is also an issue for localmh_docker). We should also keep ssh passphrases in mind, see MathHubInfo/Legacy-localmh-docker#10

[kohlhase]

@m-iancu @cmaeder fyi