Fix markup in wp-admin/themes.php. see #20403.

* Always print opening form tag.
* Always ensure table content clears to the next line. This prevents table content from bunching next to the 'Available Themes' headline, which occurred when only one theme was active on a multisite install.


git-svn-id: https://core.svn.wordpress.org/trunk@20716 1a063a9b-81f0-0310-95a4-ce76da25c4cd