Update discussion list page header

The changes in this patch are pretty much purely cosmetic so the best way to test them it to make sure all of the functionality is the same as it was in the old UI.

Test

1. Create 2 sites, one with these UI changes applied and one without.
2. Create a course with one or more forum activities (with groups modes on or off), with a few discussions in each, a few users enrolled with different roles (student, teacher)
3. Duplicate that course across both sites so the content is the same (you can create it in one and then back the course up and restore it into the other site)
4. For this test we're only concern with the functionality of the top part of the page (everything above the table that contains the discussions)
5. Confirm that all of the functionality is the same between the old UI and the new UI. Some things to check are:
   1. Creating discussions
   2. Modifying the settings
   3. Changing the visible groups
   4. Doing a basic search
   5. Doing an advanced search
      • This is accessible in the new UI via the down arrow on the right of the search form
      • This is accessible in the old UI by hitting the search button without any value in the input