Minimal warning if student has never signed in

[islemaster]

Show a much less scary confirmation dialog when removing a student who has never signed in. (spec). This is a follow-up to #23619.

Also added a story for this component since it now has multiple states:

[maddiedierker]

👍 i think we'll want to get some tests on ConfirmRemoveStudentDialog now that we're introducing additional states (rather than a basically static modal), but not a blocker for merging since this is behind a flag

[islemaster]

Hah! I actually wrote unit tests for it, then removed them in the last commit here because, from a coverage perspective, they hit exactly the same code the story does. They do make stronger assertions about the contents of the dialog though; I'm happy to put them back if you think they are useful.

[maddiedierker]

haha, i hadn't noticed that! storybook will catch if the component errors on render, but the tests will catch that the contents are correct, so i'd say add them back. if it were just the two states, i'd say leave the tests off, but since we're adding a third state if the student has their own login, having coverage on the modal content becomes more useful.