Add support for fetching gh app user. (#30)

[nishkrishnan]

Fixes the problem seen in: #1161

Atlantis running through a GH app is not configured to return a user which is necessary when hiding comments on a PR. When providing GHAppSlug, Atlantis can fetch the app name and then use that to infer the username to filter comments by. Unsure if I missed anything here or if there are unusual use cases where the user can't be tied to the app name but this seems to work for our use case.

Note, in order to ensure I'm not breaking any existing configurations I've made this feature optional. If app slug is not provided the user continues to return an empty string. If this is undesired I can make a breaking change. Will update documentation in a follow up.

[codecov]

Codecov Report

Merging #1334 (de4c966) into master (129b7b2) will decrease coverage by 0.03%.
The diff coverage is 61.90%.

@@            Coverage Diff             @@
##           master    #1334      +/-   ##
==========================================
- Coverage   70.05%   70.01%   -0.04%     
==========================================
  Files          74       74              
  Lines        5540     5556      +16     
==========================================
+ Hits         3881     3890       +9     
- Misses       1303     1307       +4     
- Partials      356      359       +3     

Impacted Files	Coverage Δ
cmd/server.go	79.09% <ø> (ø)
server/server.go	62.25% <0.00%> (-0.16%)	⬇️
server/user_config.go	100.00% <ø> (ø)
server/events/vcs/github_client.go	79.79% <50.00%> (-0.62%)	⬇️
server/events/vcs/github_credentials.go	71.83% <71.42%> (-1.51%)	⬇️

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[lkysow]

LGTM

[jamengual]

LGTM

[chenrui333]

👍