Fixes for course registration search functionality

[zootos]

Fixes two issues:

• Properly hides users that do not match search on mobile
• Hitting enter on the keyboard or equivalently the "search" button on a mobile keyboard no longer triggers a download of a spreadsheet of the selected users. Instead, it takes focus away from the search field, i.e., as if the user finished their search. This should be a natural reaction from hitting enter/"search" when searching. For the spreadsheet, the download button must be used, as would be expected.

[ddabble]

Great! 🔥