Support case-insensitive username logins

[shane-tang]

Fixes #14586. See issue for more product details.

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Changes

Previously, the login controller was searching the DB for a record with an exact case-sensitive username match (returning either a Document or undefined).

This PR makes the login controller search the DB for all records with a case-insensitive username match (returning a Document[]). If the resulting array's size is 0, user = undefined. If the size is 1, user = result[0]. If the size is 2+, we search the resulting Document[] for an exact case-sensitive username match, returning either a Document or undefined.

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Demo

Login w/out Username Collisions	Login w/ Username Collisions

Click the header link for a full video of creating the users if you want a fuller picture

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Disclaimer

I've manually tested that users with colliding usernames still log with the same case-sensitive behavior by removing the unique username constraint on my local instance's registration controller (demonstrated in this video), but I don't believe it's possible to add this case to the integration tests because we can't/shouldn't lift that constraint on the actual registration controller.

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UUID: 99f20106-7060-4398-b565-26e56c4de2ad

[CuriousMagpie]

@SabreCat This works as intended, however I'm unclear if the (case-sensitive) wording should be removed from the login page or if we want to keep it for the rare folks who have the same username but in different cases Bob vs BOB for instance.

[SabreCat]

Hmm, it's probably ok leaving it, but we can check with the rest of the team!

[saraolson]

We'd like to do an audit on the amount of actual accounts that may have a case conflict still, as we've had the lowercase username requirement for 6+ years now.