Use a nested meta query when querying by role in `WP_User_Query`.

If a user query includes a meta query together with a role argument,
nest the original meta query and append the role meta query with an
AND relationship.

fixes #23849, #27026.


git-svn-id: https://develop.svn.wordpress.org/trunk@30094 602fd350-edb4-49c9-b593-d223f7449a82
Add a hidden submit button to the Appearance -> Header screen so keyboard users can submit the form correctly when a default image provided by the theme is in use.

Fixes #20880.
Props kovshenin, joedolson.


git-svn-id: https://develop.svn.wordpress.org/trunk@30095 602fd350-edb4-49c9-b593-d223f7449a82