feat(a11y): Add keyboard accessibility to AT unit home

Changes

Make all action elements/buttons on Authoring Tool Unit Home lessons and steps are keyboard accessible.
Use MatExpansionPanel for lessons.
Disable constraint icon click action for steps in a branch path.
Change icon color for step action buttons to ensure contrast meets WCAG AA standard.

Make sure expand all and collapse all buttons in AT Unit Home work as before.
Make sure lessons can still be expanded/collapsed by clicking on lesson.
Make sure all action icons on lessons still work as before.
Make sure clicking on step still opens step authoring view.
Make sure all action icons on steps work as before.
Make sure you can use the keyboard to navigate to lessons and steps and all their action icons and the icon buttons work using the keyboard (enter key or space bar for checkboxes).

Closes #1942.