Bitbucket server integration

Created by: lguychard

This PR adds a new Webpack entrypoint in browser/: integration.tsx. It's based on phabricator/extension.tsx, but aims to be code host agnostic. The bundle, integration.bundle.js, is served as a static asset by the Sourcegraph instance.

Build tasks for copying assets (phabricator(), watchPhabricator(), etc.) have been adapted.

Rel: https://github.com/sourcegraph/bitbucket-server-plugin/pull/1

Created by: codecov[bot]

Codecov Report

Merging #4686 into master will decrease coverage by 0.05%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #4686      +/-   ##
==========================================
- Coverage   47.83%   47.78%   -0.06%     
==========================================
  Files         725      725              
  Lines       44235    44238       +3     
  Branches     1758     1758              
==========================================
- Hits        21160    21139      -21     
- Misses      21083    21106      +23     
- Partials     1992     1993       +1

Impacted Files	Coverage Δ
browser/src/libs/bitbucket/context.tsx	8.69% <100%> (ø)
browser/src/shared/util/context.tsx	50% <50%> (-2.64%)
...rowser/src/libs/code_intelligence/hover_alerts.tsx	38.88% <66.66%> (+5.55%)
cmd/frontend/graphqlbackend/textsearch.go	43.45% <0%> (-3.59%)
cmd/frontend/graphqlbackend/repository.go	9.82% <0%> (-0.18%)
cmd/frontend/db/repos.go	70.43% <0%> (+0.17%)