Add a compatibility warning banner to mobile devices

[Nightcaat]

Mobile compatibility isn't exactly our first priority, so I think that a compatibility warning should be added for mobile devices.
Here's an example of how a compatibility warning could look on an iPad:

[y21]

This issue might have been fixed in 965d165, but I'm not sure since I can only test it with my Android phone. Please visit the login page using your iPhone and tell me if a notification bar appears at the top of the page. Thank you.

EDIT: Works on Android.