Make account names case-insensitive

[aumetra]

Closes #406

---

• Names are insensitive in the accounts table
• Names are insensitive in the users table
• Convert the fields to citext (https://www.postgresql.org/docs/current/citext.html)
• Use Unicode Collation https://www.postgresql.org/docs/current/collation.html
• Adjust regex in the registration function to use \p{L}\p{N} instead of \w
• Tests added
  • accounts
  • users
  • Registration garde validator

[aumetra]

Better yet: instead of CITEXT, we can use Unicode collations. That works with things other than ASCII

https://www.postgresql.org/docs/current/collation.html

[aumetra]

Looking at the current poll results I put out on Mastodon, we will probably keep an alphanumeric check in place:

https://corteximplant.com/@0x0/111325717243948397

(yes, the results are saying "any unicode character" but the issue here is, as @tesaguri pointed out, the compatibility with the acct: URI scheme)