Form improvements

[ByordiShilpa]

https://phabricator.wikimedia.org/T306314

• "Please note!" box is #856404 text on a #fff3cd background. On a page that will be frequented by people with impaired vision.
• Below the username field, the text "Usernames may not consist entirely of numbers, contain the following characters: # / | [ ] { } < > @ % : or exceed 85 characters in length." instead of this explaining, is it possible to alert the user in an accessible way that the entered username is invalid if they enter an invalid name?
• ACC should inform that a username may not be (potentially) offensive and may not represent more than one person. This may prevent having to disclose this information whenever linking ACC on a project.
• The form elements appear to have no title attribute. Does this harm accessibility?