Improvements to filter form

This patch contains some minor improvements to the filter form on the modules page.

In Drupal 7 the padding on the filter form was missing and the submit button wasn't quiet in alignment. This patch fixes this by targeting the padding more specifically and increasing the margin on the submit button.

Also, the module filter box was showing the machine name of the modules rather than the display name. This patch fixes this by getting the system module info and using this when building the module options array.