Error handling for common mistakes on Accept Invitation username

[Paul Everitt]

(This is a nice-to-have, as KARL2 doesn't have this. However, it will cut down the common help desk requests that Nat gets.)

Put in a FormEncode validator for baseforms.username (used on Accept Invitation) to raise invalid with three different messages for:

- Check if the username *ends* in a space (special case, because it is non-visible and hard for people to report problem easily)

- Check if there are any spaces in the username

- Check for any other invalid characters (they will provide us the exact criteria later, for now, exclude anything that would require escaping in HTML)

[Shane Hathaway (shane-hathawaymix)]

Finished in revision 2736. Note that the validator already ignores any trailing spaces, so we don't need to provide an error message for that case.