Introduce sessions feature

[nunofaria11]

Instead of having the extension always running in the background, introduce the "session" feature. The idea is:

1. the ext is off by default
2. the user clicks the ext button (in browser extension panel) to start a session
3. the user does test the site/applicaiton
4. the user clicks the ext button to stop the session and download the debug artefacts

Note: this would also improve privacy matters for users.