Move the ui-extension code to another Git repo

The ui-extension code currently lives in a sub-module of the Channel Manager. Yet future UI extensions may not be limited to the Channel Manager. It's probably a good idea to move the code to a more generic place. One additional aspect to consider here Unified UI, which may (not certain yet) also use the client library.

1. Decide whether the code should indeed be moved out of the Channel Manager
2. Decide where to move to code to. Options:
   1. A sub-module of the CMS
   2. A new Git repository
   3. ...