Add avatar_url field to RepoStatus

[noahingh]

I've added the AvatarURL field to RepoStatus to bind the avatar_url field from the response. Thanks.

> Github document (combined status)

[google-cla]

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[noahingh]

@googlebot I signed it!

[gmlewis]

Thank you, @hanjunlee !
Please run go generate ./... and push (not force-push) the changes to this PR.

This is covered in our CONTRIBUTING.md document in the top of the repo.

[codecov]

Codecov Report

Merging #2021 (ba34b51) into master (a12f9cd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2021   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files         107      107           
  Lines        6901     6901           
=======================================
  Hits         6754     6754           
  Misses         81       81           
  Partials       66       66           

Impacted Files	Coverage Δ
github/repos_statuses.go	100.00% <ø> (ø)

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[gmlewis]

Thank you, @hanjunlee !
LGTM.
Merging.