Issue #160: CodeMirror modeurl wrong when using proxy prefix config (conservancy/kallithea)

[sambasama]

New issue 160: CodeMirror modeurl wrong when using proxy prefix config
https://bitbucket.org/conservancy/kallithea/issues/160/codemirror-modeurl-wrong-when-using-proxy

sambasama:

Hi.

I have Kallithea using similar configuration as in issue #159.
I have set a prefix configuration and I'm using apache as reverse proxy for Kallithea:

```
#!paster

[filter:proxy-prefix]
use = egg:PasteDeploy#prefix
prefix = /kallithea
```
Every time I add or edit gists or files and try to select the CodeMirror editor language (for ex go lang)
I see a request sent to: https://{domain}/codemirror/mode/go/go.js.
It should be: https://{domain}/kallithea/codemirror/mode/go/go.js

I think this change has something to do with the problem:
https://bitbucket.org/conservancy/kallithea/diff/kallithea/templates/admin/gists/edit.html?diff2=ccc005b96103&at=default

So this results in syntax highlighting not working / loading for selected language.