Issues when creating a quiz without having imported resources or users enrolled in the class

[pcenov]

Observed behavior

Observed while testing #12312 - if there are no imported channels and/or no imported users then we are not showing appropriate messages to the coach:

Videos:

2.No.users.match.mp4

1.No.imported.channels.-.incorrect.navigation.flow.mp4

Expected behavior

1. If there are no imported channels, when attempting to add questions users should see a modal informing them that there are no imported resources.
2. If there are no learners enrolled in the class that should also be explicitly stated.

Steps to reproduce the issue

1. Install the build from First pass string updates following UX writing and ditto review #12312
2. Attempt to add questions to a quiz without having imported any channels with exercises before that.
3. Attempt to assign the quiz to a learner without having enrolled learners in the class.

Usage Details

Windows 11, Ubuntu 22 - Chrome

[pcenov]

@radinamatic

[marcellamaki]

Strings have been added separately, UI updates will happen post-string-freeze