Added optgroup for relations done by groupby in contentypes.yml
A whole bunch of minor CSS tweaks to the backend..

pinging @peterboorsma @anketwokings
generated CSS
ResourceManager extensions url was still pointing to /app/extensions
Merge pull request #1921 from rarila/relations-groupby

groupby for relations
Merge pull request #1922 from GDmac/2.0

ResourceManager extensions url was still pointing to /app/extensions