DRY entrypoints

Created by: lguychard

Deduplicates code around extension marker and code intelligence injection in phabricator, integrations & injected script entrypoints.

Also removes comments about extension marker injection being racy in Phabricator and integration entry points: <script> tags with the defer attribute are executed after the document has been parsed, but before DOMContentLoaded fires. Meanwhile, browser extension content scripts are set to run at document_end, after DOMContentLoaded has fired.

Test Plan

• Browser extension
• Bitbucket integration
• Phabricator integration

Created by: codecov[bot]

Codecov Report

Merging #4835 into master will decrease coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4835      +/-   ##
==========================================
- Coverage   48.04%   47.97%   -0.07%     
==========================================
  Files         728      728              
  Lines       44444    44453       +9     
  Branches     1762     1762              
==========================================
- Hits        21354    21328      -26     
- Misses      21117    21148      +31     
- Partials     1973     1977       +4

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/graphqlbackend.go	38.46% <0%> (-14.69%)
cmd/frontend/graphqlbackend/search_results.go	48.54% <0%> (-1.87%)
...d/frontend/internal/authz/bitbucketserver/store.go	79.64% <0%> (-1.2%)
cmd/frontend/graphqlbackend/repositories.go	30.65% <0%> (-1.19%)
cmd/frontend/graphqlbackend/repository.go	23.03% <0%> (+0.98%)
cmd/frontend/internal/goroutine/goroutine.go	100% <0%> (+57.14%)