a11y: Make the main menu accessible

[skateman]

Keyboard navigation:

• First level a11y: Make the main menu items accessible using the keyboard #7315
• Jumping to the second level after opening it Implemented autofocus/blur to access/exit the second level menu #7467
• Hiding the second level a11y: Allow the user to exit the menu when unfocusing or ESC #7406
• Expanding/collapsing a11y: Make the menu expand/collapse accessible via keyboard #7374
• Add tooltip for left nav menu icon when menu is collapsed Fix leftnav icon tooltip and showing user info in side leftnav nav. #7415
• Add keyboard support for search Fix leftnav icon tooltip and showing user info in side leftnav nav. #7415

After all these are done, I think we should do a little cleanup.

[skateman]

@kavyanekkalapu you mentioned on the last meeting that the second level of the menu is not accessible, this is not completely true, the problem is that it is accessible only after going through all the elements in the first level. This is because the second level isn't interleaving into the existing menu structure, but we create a new one. The straightforward solution would be to have a nested markup and sort out the styling to look like it looks now, but I'm not sure if it's doable.

This would also make the hiding the second level easier, because it's easier to deal with onFocus back to the first menu than to onBlur to anything else 😕

cc @himdel and maybe @Ashley-Johnson28