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.).
@mattpap I guess what I am actually wondering is can #554 be closed by this PR? Will bokehjs be able to coexist in the notebook with other libs, etc.