Signup mechanism shortens usernames without warning

[hughcharlesparker]

When a user signs up for an account on a wordpress blog, if their chosen username is longer than the limit, wordpress chops of the end of the username, without warning the user, and without offering the user the opportunity to choose again.

Steps to reproduce: Go to a wordpress blog, sign up with a long username, and read the confirmation email. An example: forum.xbmc.org, which has a limit of 15 characters.

Insert maxlength attribute into user_login field to prevent long usernames being truncated silently

I think we're missing some maxlength attributes - these would prevent someone submitting a too-long username in the first place. Elsewhere (e.g., in show_user_form()) they are present.

Also the maxlength value for email was wrong - user_email accepts varchar(100).

Username values related: #5918, #17793