Differentiate between invalid and missing admin emails when adding a new site

Check the emptiness of the admin email before using `sanitize_email()` and `is_email()` to determine if the address is valid.

Fixes #17890


git-svn-id: https://develop.svn.wordpress.org/trunk@29877 602fd350-edb4-49c9-b593-d223f7449a82
Quicktags: remove `accesskey` attributes the buttons and don't add empty `title` attributes, fixes #29358

git-svn-id: https://develop.svn.wordpress.org/trunk@29878 602fd350-edb4-49c9-b593-d223f7449a82
Fix support for the `hide` option in wpColorPicker, props tollmanz, fixes #29632

git-svn-id: https://develop.svn.wordpress.org/trunk@29879 602fd350-edb4-49c9-b593-d223f7449a82
Do not send a welcome notification when noconfirmation has been flagged

When adding a new user to a site on the network, 2 emails are sent out - one for confirmation and one as a welcome. If the option for no confirmation is selected, neither should send.

props transom
Fixes #16235


git-svn-id: https://develop.svn.wordpress.org/trunk@29880 602fd350-edb4-49c9-b593-d223f7449a82