Fix externalURLs alert logic

Created by: felixfbecker

Fixes https://github.com/sourcegraph/customer/issues/118

The issue was that the code assumed the array always had an element, but it doesn't need to have. I also noticed that we didn't check whether we actually support the code host. With this PR, we take the first code host from external links that we support, and show nothing if there is none.

cc @dadlerj

Created by: codecov[bot]

Codecov Report

Merging #14980 into main will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main   #14980   +/-   ##
=======================================
  Coverage   52.34%   52.34%           
=======================================
  Files        1556     1556           
  Lines       79540    79550   +10     
  Branches     7137     7044   -93     
=======================================
+ Hits        41635    41642    +7     
- Misses      34157    34159    +2     
- Partials     3748     3749    +1     

Flag	Coverage Δ
#go	52.30% <ø> (-0.01%)
#integration	30.32% <6.25%> (-0.02%)
#storybook	25.82% <93.75%> (+0.04%)
#typescript	52.44% <100.00%> (+0.02%)
#unit	34.12% <100.00%> (+0.02%)

Impacted Files	Coverage Δ
.../src/repo/actions/InstallBrowserExtensionAlert.tsx	100.00% <100.00%> (ø)
.../internal/codeintel/resolvers/graphql/locations.go	83.50% <0.00%> (-2.07%)
cmd/repo-updater/repos/types.go	74.20% <0.00%> (-0.25%)

Created by: felixfbecker

@sourcegraph/distribution please include this in a patch release when this is merged, thanks!