Remove TimeCardObject.submitted

[Jkrzy]

Description

Resolves: #881 by removing TimeCardObject.submitted and replacing references in queries/tests to the associated TimeCard object's submitted field.

[codecov-io]

Codecov Report

Merging #933 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
- Coverage   91.47%   91.45%   -0.03%     
==========================================
  Files          39       39              
  Lines        1689     1685       -4     
==========================================
- Hits         1545     1541       -4     
  Misses        144      144

Impacted Files	Coverage Δ
hours/views.py	88.2% <0%> (-0.07%)	⬇️
hours/models.py	96.21% <0%> (-0.05%)	⬇️
hours/admin.py	71.21% <0%> (ø)	⬆️

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[cantsin]

Tried: toggling submitted status on timecard using admin, submitted timecard as user, unsubmitted that timecard, ran reports with and without submitted timecard, and tested the API likewise. All LGTM with the caveat that I'm still new to this codebase.