Revert invalid group greying in Set Quiz modal

[sjd210]

A change here added a front-end warning and disabling of invalid groups (for which the quiz had already been set) in the dropdown menu.

This reverts the disabling in the dropdown menu (since it was unclear what was happening) but keeps the front-end warning.

(I also noticed that the check for passing due date was checking for the past instead of the future so that's been fixed here too)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 36.48%. Comparing base (1419464) to head (fbbb9b8).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
...pp/components/elements/modals/QuizSettingModal.tsx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
+ Coverage   36.43%   36.48%   +0.05%     
==========================================
  Files         427      427              
  Lines       19066    19039      -27     
  Branches     6281     5632     -649     
==========================================
  Hits         6947     6947              
- Misses      11488    12054     +566     
+ Partials      631       38     -593     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.