Fix TinyMCE asset_proxy by stripping STATIC_URL, leading / from proxied URLs

`STATIC_URL` often contains `host` or `generic_host` (esp. if `STATIC_URL` is a
path on the same domain), so it needs to be removed first to ensure it
is removed completely. Also removed leading `/` from URL, since it
appears staticfiles doesn't like absolute paths, and I suspect that's what was causing the `SuspiciousOperation`.

This fixes #779 for me.