Unit-testable click.js

[PromyLOPh]

click.js should be unit-testable, so we can easily identify breaking layout changes.

• [x] Move the sites object to the Python world
• [x] Add tests that check for selector existence and whether an event handler is attached or not
• [ ] (opional) Check that a click does indeed result in the expected action (network request, element replaced, …)