UI should indicate and prevent duplicate task for same benchmark and para

[canjalal]

ACTUAL

If case manager attempts to assign a benchmark to Para "A", then tries to assign the benchmark to Para "A" once more, the UI will allow them to (checkbox is empty next to name).

Furthermore, instead of showing an error, the app allows the case manager to actually create a duplicate task assigned to Para "A". This results in a duplicate row in the tasks table.

EXPECTED

If case manager attempts to assign a benchmark to Para "A", then tries to assign the benchmark to Para "A" once more, the UI will show a checkbox next to their name b/c they are already assigned. The case manager can then choose to uncheck or recheck the checkbox to unassign or reassign.

Furthermore, if the front end UI checks are broken or bypassed, the app should not allow the case manager to actually create a duplicate task assigned to Para "A". Duplicate rows in the tasks table should not be created no matter what.

[canjalal]

Blocked by #439

[canjalal]

Do we want to be able to unassign benchmarks? If we do this, it will destroy the data on the due_date, trial_count, and the seen flag.