Don't reference Bootstrap alert in HoverOverlay

Created by: felixfbecker

Fixes #6151

Tested:

• Phabricator
• Sourcegraph web app.
• GitLab
• GitHub

Created by: lguychard

@felixfbecker pushed up a commit to use GitHub's flash alert classes:

Created by: felixfbecker

Oh, does GitHub not have alerts?

Created by: codecov[bot]

Codecov Report

Merging #6162 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #6162   +/-   ##
======================================
  Coverage    39.8%   39.8%           
======================================
  Files        1169    1169           
  Lines       58120   58120           
  Branches     5739    5739           
======================================
  Hits        23137   23137           
  Misses      32849   32849           
  Partials     2134    2134

Impacted Files	Coverage Δ
browser/src/libs/phabricator/code_intelligence.ts	49.31% <ø> (ø)
browser/src/libs/github/code_intelligence.ts	57.03% <ø> (ø)
browser/src/libs/bitbucket/code_intelligence.tsx	62.82% <ø> (ø)
web/src/components/shared.tsx	76.92% <ø> (ø)
browser/src/libs/gitlab/code_intelligence.ts	60% <ø> (ø)
shared/src/hover/HoverOverlay.tsx	70.17% <100%> (ø)