Disable 'continue' button during quiz creation unless a selection is made

[marcellamaki]

Observed behavior

During the Uganda training, while creating a quiz, many participants clicked on ‘continue’ after entering the title and the number of questions instead of clicking on the channel first

Expected behavior

A user should not have the option to click the "continue" button unless they have selected content needed. It could perhaps be disabled, or the bottom bar could only show up when the content has been selected

User-facing consequences

Confusion about how to follow the workflow.

Steps to reproduce

Add a title and selected number of questions. Click the "continue" button before selecting any content.

[rtibbles]

Fixed in #9515