Add an ENV var for extra css

[nlsvgtr]

PR https://github.com/openstad/openstad-oauth2-server/pull/119/files updates implementation specific design in the generic repo. This specific code should not be in the repo in the first place.

An env var that points to an external stylesheet could fix this, and is consitent with the way this is solved in the frontend..

[Badmuts]

I believe this already available: https://github.com/openstad/openstad-oauth2-server/blob/master/routes/routes.js#L156

[nlsvgtr]

Then let me refrase this issue: remove Amsterdam specific CSS from the repo and move it to env.STYLESHEETS