[WIP] TA Timers

[kevc528]

This PR allows TA's to create warning timers for a course. These timers will be membership specific so different TA's can have different timer values (or no timers at all).

[codecov[bot]]

Codecov Report

Merging #171 (d330a1c) into master (863a2fe) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   98.12%   98.13%           
=======================================
  Files          24       24           
  Lines        1226     1231    +5     
=======================================
+ Hits         1203     1208    +5     
  Misses         23       23           

Flag	Coverage Δ
backend	98.13% <100.00%> (+<0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
backend/ohq/models.py	100.00% <100.00%> (ø)
backend/ohq/permissions.py	99.10% <100.00%> (+0.01%)	:arrow_up:
backend/ohq/serializers.py	100.00% <100.00%> (ø)

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