Username case fields

[stuartpb]

Things to list:

• username.case.sensitivity: should be "insensitive", can be "sensitive" if the system is stupid (accepts "stuart" and "Stuart" as two different users).
• username.case.display: Whether the system coerces usernames to "upper" or "lower", or keeps them "mixed".

As for passwords, if the password is case-insensitive, that is a HUGE red flag, and it belongs in redflags (since it's not common).

[stuartpb]

Putting this on future, even though it could feasibly be added before v0.1.0.

[stuartpb]

There's also the matter of how they're handled in URLs, and in what contexts they're corrected (ie. rewritten in mentions, redirected in links).

[stuartpb]

Right now, systems coercing usernames to (ie) lowercase in all instances can be represented as username.contents.whitelist.classes[0] = 'lowers', FWIW (though that's admittedly not a great way of representing all these nuances).

[stuartpb]

Moving to opws/opws-schemata#25 (per #308).