Issue 3410 disable continue

@jedp - they are coming in fast and furious!

Disable "continue" button on mobile until the user has typed in a character.

    * On mobile, rename the next button to continue
    * Disable the "continue" button until the user types a char in the email field.
    * Clean up the button hover states on desktop and mobile when the button is diabled.

This builds upon #3408, so that should be reviewed first.

The ephemeral instance to test on is disable-continue.personatest.org