Administration: Improve the usage of the button CSS classes.

Introduces some consistency in the usage of the button CSS classes, fixes the
focus style for accessibility and responsiveness of the buttons.

- Adds the `button` class to all primary buttons make them responsive
- Removes all `secondary-button` classes and replaces it with button when needed. `button-secondary` shouldn't be used and exists just for backward compatibility reasons
- Replaces classes inside `submit_button()` with a shorthand for some buttons, and use an empty string for the default `button` class. Passing `button` is unnecessary
- Adjusts `get_submit_button()` to remove empty items

Props iseulde, dimchik, chris_d2d, mhowell, afercia.
Fixes #27314, #37138, #37448.


git-svn-id: https://develop.svn.wordpress.org/trunk@38672 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/ie.css

@@ -221,8 +221,7 @@ table.fixed td {
 }
 
 #wpbody-content input.button,
-#wpbody-content input.button-primary,
-#wpbody-content input.button-secondary {
+#wpbody-content input.button-primary {
 	overflow: visible;
 }
 
@@ -447,8 +446,8 @@ div#dashboard-widgets {
 	display: block;
 }
 
-.tablenav .button-secondary,
-.nav .button-secondary {
+.tablenav .button,
+.nav .button {
 	padding-top: 2px;
 	padding-bottom: 2px;
 }
@@ -507,7 +506,7 @@ table.ie-fixed {
 	border-collapse: collapse;
 }
 
-.tablenav a.button-secondary {
+.tablenav a.button {
 	display: inline-block;
 	padding: 2px 5px;
 }