Added type checking to trigger model

[hinashi]

When using ForeignKey in a Django model, there was an issue where relation_name completion was not working.

Added type hints based on the issue below.
typeddjango/django-stubs#579 (comment)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.83673% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 96.66%. Comparing base (8df9939) to head (a8fd401).

Files	Patch %	Lines
trigger/models.py	91.48%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1108      +/-   ##
==========================================
+ Coverage   92.08%   96.66%   +4.58%     
==========================================
  Files         319      193     -126     
  Lines       27753    23065    -4688     
  Branches     1242        0    -1242     
==========================================
- Hits        25555    22296    -3259     
+ Misses       2098      769    -1329     
+ Partials      100        0     -100     

Flag	Coverage Δ
core	96.66% <91.83%> (-0.02%)	⬇️
ui	?

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[userlocalhost]

Thank you for improving 👍