Implement scrollBoundaries for GitHub, GitLab

[lguychard]

Rel RFC 76
Depends on sourcegraph/codeintellify#196

[codecov]

Codecov Report

Merging #7023 into master will decrease coverage by <.01%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #7023      +/-   ##
==========================================
- Coverage   39.52%   39.52%   -0.01%     
==========================================
  Files        1241     1241              
  Lines       65216    65228      +12     
  Branches     6108     6112       +4     
==========================================
+ Hits        25777    25779       +2     
- Misses      37083    37093      +10     
  Partials     2356     2356

Impacted Files	Coverage Δ
browser/src/libs/code_intelligence/code_views.ts	90.47% <ø> (ø)	⬆️
browser/src/libs/github/code_intelligence.ts	55.31% <0%> (-2.04%)	⬇️
browser/src/libs/gitlab/code_intelligence.ts	55.17% <20%> (-3.32%)	⬇️
...r/src/libs/code_intelligence/code_intelligence.tsx	65.59% <50%> (-0.15%)	⬇️