When installing existing assessment, don't autoselect all

[jhs-panda]

Requested by 15213.
When importing assessment from file system, currently all assessments will be autoselected to be imported. Not having them all be autoselected would be nice.

https://docs.autolabproject.com/lab/#importing-an-autograded-lab-from-a-previous-semester
Proposed solution: include a button on top to autoselect all, but otherwise have them not be autoselected to begin with.