Configure codecov status checks in PRs

[Splines]

Status checks: https://docs.codecov.com/docs/commit-status

Validity of schema checked via:

curl --data-binary @./.config/.codecov.yml https://codecov.io/validate

[codecov]

Codecov Report

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

Project coverage is 66.64%. Comparing base (6bde35d) to head (a6bd2be).

Files	Patch %	Lines
app/helpers/talks_helper.rb	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #663   +/-   ##
=======================================
  Coverage   66.64%   66.64%           
=======================================
  Files         308      308           
  Lines        9386     9386           
=======================================
  Hits         6255     6255           
  Misses       3131     3131           

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