Add additional context information to UI

[jlesquembre]

See #43

I'm not sure about what to write on the info/help section, before merging I need to write something useful here. Suggestions are very welcome.

nixpkgs version is hard-coded, first we need to add an endpoint to the API to provide that data. I'm not sure, but I think the version is not currently saved on the DB.

Now looks like this, suggestions about the design are welcome too:

[jlesquembre]

Ok, I'm merging it, the remaining changes will be done in another PR