Make the new "Insert an activity or resource before X" more accessible

In MDL-75596 a new option was added to the course page to let teachers add content anywhere in a section, not just at the end.

The UX team has reviewed the current behaviour and created a prototype to make it more accessible: https://www.figma.com/proto/VSt2KoA3Rabjvtf7dgckRE/Add-activities-or-resources-anywhere-in-a-section?page-id=1%3A1871&node-id=2%3A2065&viewport=1652%2C498%2C0.27&scaling=min-zoom&starting-point-node-id=2%3A2065 

A few things to consider:

• The 'Add activity or resource' floating buttons will be triggered when hovering activity cards (right now it's only displayed on hover in the space between the activity cards). The floating button will appear on the top of the hovered activity. 

• The prototype doesn't reflect the activity card hover style when hovering as Figma has limited prototyping possibilities, but the idea is that by hovering an activity, the card will change to the hover state (dark blue outline, light blue fill).