make usernames case-insensitive, sanitize inputs against illegal characters

[jerkey]

the system is presently case-sensitive when it comes to usernames, and this is a problem because people are not expecting this.

phones (and some other platforms) implement an automatic capitalization of the first character typed into a lot of fields, and maybe this is why my username in the system has the first letter capitalized? I certainly didn't do that on purpose.

This change needs to be done simultaneously with a review of the user database, to de-capitalize any usernames and also to make sure that there are no conflicts such as multiple accounts with the same spelling, differing only by capitalization.

[kenrestivo]

Looks like it uses https://www.npmjs.com/package/accountdown