ep_syntaxhighlighting: use relative urls

Hi,

the urls for the scripts and stylesheets in ep_syntaxhighlighting were absolute (/static/...) instead of relative so they didn't load on an etherpad installed behind a reverse proxy and mounted in a subdirectory.

This pull request changes all absolute urls in ep_syntaxhighlighting to relative ones.

Have a nice day,
~robbi5