Users: Add id="user-{ID}" to rows on WP_MS_Users_List_Table.

Brings the network Users list in line with `WP_Users_List_Table::single_row()`,
which has always emitted `<tr id="user-{ID}">`. Without a row id, plugin and
assistive-tech code that targets specific rows has to fall back to walking
from the bulk-action checkbox (`#blog_{ID}`) up via `.closest('tr')`.

Additive: existing consumers of the `class` attribute are unaffected.

See https://core.trac.wordpress.org/ticket/65102.

Author: Dion Hulse

src/wp-admin/includes/class-wp-ms-users-list-table.php

@@ -502,7 +502,7 @@ public function display_rows() {
 			}
 
 			?>
-			<tr class="<?php echo trim( $class ); ?>">
+			<tr id="user-<?php echo (int) $user->ID; ?>" class="<?php echo trim( $class ); ?>">
 				<?php $this->single_row_columns( $user ); ?>
 			</tr>
 			<?php