[bug:ui] Preview keyboard keys do not work anymore

[nemesifier]

Open any config object like a Device, Template or VPN Server.

Make sure there's some config defined.

Problem 1: Opening Preview

Try opening the preview with the ALT+P keys.

Expected autcome: preview overlay opens.
Actual outcome: nothing happens.

Problem 1: Closing Preview

Try opening the preview by clicking on the preview button.
Now press ESC key to close.

Expected autcome: preview overlay closes.
Actual outcome: nothing happens.

Automated tests

We need to continue making OpenWISP more robust and prevent these UI bugs from happening again.
Fortunately, we have already added some Selenium UI tests here:
https://github.com/openwisp/openwisp-controller/blob/master/openwisp_controller/config/tests/test_selenium.py

We must write 2 new tests which replicate these issues: we must ensure the tests fail with an understandable error message before applying the fix.

[nemesifier]

Seems to be an interaction with openwisp/openwisp-notifications#275, nonetheless, I still think it's good to add these selenium tests.