Refactor the extensions infrastructure about fritz

[stefanv]

I've now investigated Stacked Git, and I don't think it gives us much advantage over rebasing.

What I suggest we do:

1. Pin skyportal in fritz to fritz-marshal/skyportal
2. Maintain a list of patches as individual branches on top of that repo. In other words, we have branches patches/dockerfile, patches/config etc.
3. The master branch of fritz-marshal/skyportal is constructed with a so-called octopus merge: git merge patches/dockerfile patches/config. We provide a tool to automate updating SkyPortal and doing the octopus merge, so a fritz dev can simply type: fritz skyportal update to bring in latest master or fritz skyportal apply-patches to apply patches to a vanilla SkyPortal.
4. We then push the new (merged) branch to fritz-marshal/skyportal and pin.

The patch branches should each address one feature, and be self contained. The patch branch is updated as normal, by adding git commits.

The advantage of this schema over simply keeping patches on top of SkyPortal itself (which would require frequent rebases and rewriting of patches) is that we get to track the history of changes made to SkyPortal.