Do not allow YouTube video URLs when using the URLPicker

[acelaya]

Since we are currently using raw YouTube video URLs for video assignments, there's a possibility that someone configures it using the regular URLPicker instead of the YouTubePicker.

This is problematic, as the latter performs some extra validation that is not performed by the URLPicker (is a public existing video, it's not age-restricted, it's embeddable, etc).

Bypassing these validations could result in an assignment which fails for students at a later point in time.

This PRs adds a simple check that displays an error if a YouTube video URL is set in the URLPicker. The message is different if youtube is not allowed for the active course.

YouTube enabled:

YouTube not enabled:

Considerations / TODO

• There are other possible approaches to address this problem. They were listed in Handle assignments created with a YouTube URL using the regular URLPicker #5499, but this seems like the simplest way to "get away with it" for now.
• We need to define an error which is meaningful enough. I considered mentioning what to do next, e.g. "Go back and select YouTube instead", but considering YouTube is enabled/disabled by course, this might be the wrong assumption.
• Related to the above, we might consider letting YouTube video URLs through, when YouTube is not enabled for the course, or dynamically display different error messages.

This PR closes #5499

[robertknight]

We need to define an error which is meaningful enough. I considered mentioning what to do next, e.g. "Go back and select YouTube instead", but considering YouTube is enabled/disabled by course, this might be the wrong assumption.

I believe the plan is to enable YouTube for everyone once the feature is released, so this will no longer be a problem. What we could do in the interim is to show different messages depending on whether the flag is enabled or not.

If enabled:

"To annotate a video, go back and choose the YouTube option."

If disabled:

"Annotating YouTube videos is not yet supported. This feature is coming soon."

[acelaya]

I believe the plan is to enable YouTube for everyone once the feature is released, so this will no longer be a problem. What we could do in the interim is to show different messages depending on whether the flag is enabled or not.

If enabled:

"To annotate a video, go back and choose the YouTube option."

If disabled:

"Annotating YouTube videos is not yet supported. This feature is coming soon."

I like it!

[robertknight]

LGTM