Allow the discovery of search implementation UIs in the Search UI

Right now, the search admin UI uses a static list to define the currently available search implementations that an admin can choose from. To add a new search implementation UI, one must edit the static list definition inside the class definition and add the new implementation.

We need a way to dynamically discover new implementations at runtime, depending on what extensions are installed in the wiki. Most likely, the UI extensions module could be used to achieve this.