Admin notices: Add more variety and make more generically usable.

There is now a base class of `notice`, with additional classes of `notice-success`, `notice-warning`, `notice-error`, and a new blue `notice-info`.

Also corrects some misleading notice colors, such as plugin tested up to warnings and login messages.

props avryl, melchoyce.
fixes #27418.


git-svn-id: https://develop.svn.wordpress.org/trunk@30505 602fd350-edb4-49c9-b593-d223f7449a82