Make card hover effect for exam/documents match that of categories

I personally like the current homepage's hover effects on the category cards, not too jarring but clear enough.

On the contrary, exam cards and document cards had no hover effects, which imo made the page feel a bit stationary and unresponsive to user actions. This MR fixes that (see screenshot below of a hover effect).

In addition, two small features have been implemented:

• Reduce the content shift on category pages when navigating back from an exam -- this is done by positioning the Loader component absolutely and in the corner instead of in the middle, which previously shifted the whole exam/document list down for a brief moment
• Easier multi-selection of exams for download -- after ticking one exam for download, you can now click on anywhere on other exams to toggle their selection, instead of aiming at the small checkboxes. This makes it less likely to accidentally navigate to an exam and lose the selection.