Added Google Analytics to the editor

This PR contains a rudimentary implementation of Google Analytics tracking in the Editor. For now, only tracks users starting/ending a session with the editor. We need to carefully design how we are gonna use this.

Also, I added a new service to bugr, to activate installations. The first time the editor starts, is activated calling to bugr, and receives back an GUID that is stored in a preference and used in Google Analytics, but that can also be used with other purposes.