Limit username field to 20 characters

The username field allows 24 characters.
Usernames cannot be longer than 20 characters.
I pasted my usual username, only to be told it was too long.
This is unexpected behaviour of a field that has a max length.

Solution: Limit the field to 20 characters.
* When at 20 characters and another key is pressed, display the string "Max Length" instead of accepting the character and displaying the string "Too Long"
* When a string longer than 20 characters is pasted, accept the first 20 characters and display the string "Max Length" instead of accepting 24 characters and displaying the string "Too Long"