Fixes for Mopidy webclient

I was trying to install Mopidy webclient in Raspbian with Mopidy installed via apt. This pull request fixes the problem I bumped into with the websocket crashing when given an url with double ports like: 'http://192.168.1.66:6680:6680/mopidy/ws'. In the PR I also included some additions for the manual installation instructions to fix the symbolic link to mopidy installation (or more specifically, mopidy.min.js).

BR, Jaakko