Make displaying of missing submissions on gradesheet a toggleable option

[damianhxy]

The gradesheet is used by some classes to assign grading ranges to TAs.

2019 began including missing students by default which complicates the process since instructors have to sort missing submissions to the start / end. This makes the process error-prone and also causes the numbering to diverge between the gradesheet and "view source" page (which only counts actual submissions).

Thus, we should make this a toggleable option (disabled by default) to effectively revert to the old behavior.

Note: regardless, the gradesheet / view source pages always counted submissions by instructors / CAs / dropped students. If there will be a significant amount of submissions by them, further action might be needed to make the distribution of work fairer.

[damianhxy]

Possible implementation: make use of a query_string param, add button to toggle.

Also related: sorting on problem scores is broken. Seems to be due to missing submissions not having problems.