Expand technical restrictions warning

[mrsrec]

Pull request #194 added the use of a built-in message when a username starts with a lowercase letter. This functionality has been expanded. Now, there are custom messages, and they will show more often. There is a message for when a username uses impossible underscores, and a message for when a username begins with any letter, uppercase or lowercase. Instead of updating once, it updates as you type, so you can preview how it will look on-wiki. This is important to let people know that they can change the appearance of any letter, except the first one. This lets you see how it will look onwiki.

Closes #200

[mrsrec]

I added the text, but forgot the part where it...actually accepts different cases. Oops. I will add that soon.

[mrsrec]

Tested and now working.

[jacob-g]

Looks good, tested on my local and works fine. In particular I made sure to test you can't get two active requests with the same username and different capitalization (e.g. "JVVG" and "jvvg"), and the existing database logic already takes care of that.