Accessibility: Improve settings tables and forms after [45403].

- adds `role="presentation"` to the edit comment table 
- removes a few pointless `<fieldset>` elements
- adds a few missing `<label>` elements
- adds the CSS class `class="td-full"` to table rows spanning to multiple cells
- adds explicit `scope="row"` attribute to the table headers in `options-permalink.php`: this table is better communicated as data table 
- uses consistent label association in the "Privacy Settings" page
- in the installation page "Set up your database connection": associates descriptions to their inout fields using `aria-describedby`
- improves the link to gravatar.com in the `user-edit.php` page

See #46899.
Fixes #47390.


git-svn-id: https://develop.svn.wordpress.org/trunk@45416 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-comment.php

@@ -43,7 +43,7 @@ if ( 'approved' === wp_get_comment_status( $comment ) && $comment->comment_post_
 <h2 class="edit-comment-author"><?php _e( 'Author' ); ?></h2>
 <fieldset>
 <legend class="screen-reader-text"><?php _e( 'Comment Author' ); ?></legend>
-<table class="form-table editcomment">
+<table class="form-table editcomment" role="presentation">
 <tbody>
 <tr>
 	<td class="first"><label for="name"><?php _e( 'Name' ); ?></label></td>