Add support for GitHub review webhooks

Created by: eseliger

This works on #7998 (closed). Stacked on top of https://github.com/sourcegraph/sourcegraph/pull/14742, this PR adds support for the GitHub webhook events required to support real-time draft mode changes.

Created by: codecov[bot]

Codecov Report

Merging #14862 into main will increase coverage by 3.54%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #14862      +/-   ##
==========================================
+ Coverage   48.65%   52.20%   +3.54%     
==========================================
  Files        1555     1555              
  Lines       79496    79516      +20     
  Branches     7107     7006     -101     
==========================================
+ Hits        38677    41509    +2832     
+ Misses      37072    34260    -2812     
  Partials     3747     3747              

Flag	Coverage Δ
#go	52.37% <100.00%> (+0.02%)
#integration	30.51% <ø> (?)
#storybook	22.16% <ø> (ø)
#typescript	51.76% <ø> (+12.16%)
#unit	33.70% <ø> (ø)

Impacted Files	Coverage Δ
enterprise/internal/campaigns/webhooks.go	43.02% <100.00%> (+2.26%)
enterprise/internal/campaigns/state.go	78.19% <0.00%> (+0.93%)
client/web/src/search/results/SearchResults.tsx	58.13% <0.00%> (+1.55%)
...lient/web/src/search/results/SearchResultsList.tsx	73.75% <0.00%> (+1.87%)
client/web/src/org/area/OrgInvitationPage.tsx	2.08% <0.00%> (+2.08%)
client/web/src/site-admin/backend.tsx	3.66% <0.00%> (+2.75%)
client/web/src/nav/NavLinks.tsx	87.50% <0.00%> (+3.12%)
...se/campaigns/close/CampaignCloseChangesetsList.tsx	82.75% <0.00%> (+3.44%)
client/web/src/global/GlobalAlert.tsx	3.57% <0.00%> (+3.57%)
client/web/src/search/helpers.tsx	96.55% <0.00%> (+4.31%)
... and 194 more