Configure codecov

[mickel8]

• disabled github notifications that something is not covered by tests
• we don't need to wait for other checks to run codecov
• changed the layout of codecov comment so we see more info. Behavior default means that the comment is updated when new commits appear
• disabled coverage threshold

[codecov]

Codecov Report

Merging #13 (d32a1aa) into master (1aeef5b) will increase coverage by 0.64%.
Report is 1 commits behind head on master.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   74.12%   74.77%   +0.64%     
==========================================
  Files           9        9              
  Lines         344      333      -11     
==========================================
- Hits          255      249       -6     
+ Misses         89       84       -5     

Files	Coverage Δ
lib/ex_webrtc/session_description.ex	0.00% <ø> (ø)
lib/ex_webrtc/peer_connection.ex	74.23% <66.66%> (-0.18%)	⬇️
lib/ex_webrtc/peer_connection/configuration.ex	66.66% <66.66%> (+16.66%)	⬆️

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[LVala]

I was about to do similar changes, but it seems that this time I won the "be very annoyed and do nothing about it" contest.