Control-Enter in page popup to save and close

For me, *by far* the most common page popup use case is to add or remove a couple of pass keys.  This PR:
- Focuses the pass keys text input when the popup is opened.
- Activates both text inputs such that `Control-Enter` saves changes and closes the popup.

For other use cases (such as removing a rule), no extra keystrokes/clicks are required.

With this change, you can add or remove pass keys without touching the mouse.*

<sub>(* Assuming you have a chrome shortcut to open the vimium popup.)</sub>