Fix server styles after PR #550

This PR restores  `bokeh-sever`'s look from before PR #550, well approximately, because now `bokeh-server` uses bootstrap 3. Also cleaned up server's templates a bit (more inheritance, etc.).
Yep, In d4b44c020ba874871719a0b3b2591ccc272810c9 to `src/coffee/common/custom.coffee` (your commit btw.).