Ferdi: Ferdi email login should be case insensitive

[mattdeitke]

Bug description: Ferdi email login should be case insensitive.

Steps to reproduce: Simply try signing in with Ferdi using a different email casing (e.g., [email protected] vs. [email protected]) and you'll notice that only the casing you used when signing up works.

Expected behavior All casing variations of an email are equivalent when logging into Ferdi.

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[vantezzen]

Thank you for your bug report. We are already discussing this at https://github.com/getferdi/server/issues/24