Improvement: Allow disconnect from 'Connecting', with limits.

[lesleyjanenorton]

This PR:

• Disables the toggle when the extension is connecting.
  Why:
  • Button mashing on the toggle when the extension is 'connecting' (meaning the VPN client is transitioning from StateOn to StateOnPartial) results in wonky/unexpected behavior.
  • The toggle already appears disabled when connecting. You wouldn't expect clicking on it to do anything.
• Adds a 15 second timeout, after which the extension toggle is re-enabled (actually and visibly)
  Why:
  • If the toggle remains disabled, and the client never successfully transitions to 'StateOnPartial', the extension is stuck in the connecting state.