Admin style updates

[iammattthomas]

Lots of updates to the admin styles here

• Added secondary button class per convo with Liz; applied button-secondary class in all pages as needed
• Updates to header positioning to more closely match HC comps
• Fix issue with footer causing a vertical scrollbar even on short pages
• Applied new table styles to forms where appropriate

[iammattthomas]

Patches items listed in ticket

[ryan]

(In [6950]) Admin styling updates from mt. fixes #5940