Fix GitHub snippet resolution

Created by: lguychard

Fixes #771 Fixes #3105 Fixes #3110

Deprecates commentSnippetCodeViewResolver, which:

• Matched entire comments, even though they may have contained several snippets (#3110)
• Used the first link whose href could be parsed by getGitHubState(), which could be any inline link in a GitHub comment (#771)

Snippet code views are now resolved using the fileLineContainerResolver, to the closes element containing both the code and the permalink. Several snippets contained in the same comment can be accurately resolved.

Test Plan

Code Hosts

• GitHub
• GitHub Enterprise
• Refined GitHub
• Phabricator
• Phabricator integration
• Bitbucket
• Gitlab

Browsers

• Chrome
• Firefox

Created by: codecov[bot]

Codecov Report

Merging #3920 into master will decrease coverage by 0.1%. The diff coverage is 25%.

Impacted Files	Coverage Δ
browser/src/libs/github/util.tsx	29.62% <ø> (+2.22%)
browser/src/libs/github/code_intelligence.ts	51.26% <0%> (-1.74%)
browser/src/libs/github/file_info.ts	21.95% <30.76%> (ø)
cmd/frontend/backend/go_importers.go	22.22% <0%> (-37.5%)
cmd/frontend/backend/repos_vcs.go	22.03% <0%> (-7.63%)
cmd/frontend/backend/repos.go	28.71% <0%> (-2.98%)
cmd/repo-updater/repos/migrations.go	69.66% <0%> (-0.71%)

Created by: felixfbecker

What do you mean with "deprecates"? Why can't we remove it?

Created by: lguychard

@felixfbecker poor wording on my part, it is removed, the useful bits are kept in snippetCodeView.

Created by: felixfbecker

@lguychard I think this is fine to merge since it doesn't regress on anything, it's a net improvement, right?

Created by: lguychard

Yeah, I was holding off for !3976 (closed), but we can merge without it.