Add support for draft changesets in reconciler

Created by: eseliger

This PR adds support for creating and undrafting PRs on GitHub to the reconciler. It is the main chunk of work missing towards https://github.com/sourcegraph/sourcegraph/issues/7998, before I can start GitLab support, and some minor UI/GQLAPI things.

I tried making the commits as descriptive as possible, and I would strongly suggest to review this PR commit-by-commit, all at their own are pretty simple to understand, but they are all three needed to form a working reconciler implementation that works, hence I made them be one PR, consisting of 3 pieces.

Created by: codecov[bot]

Codecov Report

Merging #14807 into main will increase coverage by 3.55%. The diff coverage is 69.23%.

@@            Coverage Diff             @@
##             main   #14807      +/-   ##
==========================================
+ Coverage   48.65%   52.20%   +3.55%     
==========================================
  Files        1555     1555              
  Lines       79496    79563      +67     
  Branches     7107     7062      -45     
==========================================
+ Hits        38677    41536    +2859     
+ Misses      37072    34278    -2794     
- Partials     3747     3749       +2     

Flag	Coverage Δ
#go	52.38% <69.23%> (+0.02%)
#integration	30.50% <ø> (?)
#storybook	22.16% <ø> (ø)
#typescript	51.76% <ø> (+12.15%)
#unit	33.70% <ø> (ø)

Impacted Files	Coverage Δ
cmd/repo-updater/repos/sources.go	63.82% <ø> (ø)
internal/campaigns/changeset.go	49.86% <0.00%> (+0.40%)
internal/campaigns/util.go	38.46% <0.00%> (-7.00%)
cmd/repo-updater/repos/github.go	63.68% <50.00%> (-2.00%)
cmd/repo-updater/repos/bitbucketserver.go	68.96% <60.00%> (-1.12%)
enterprise/internal/campaigns/reconciler.go	60.95% <71.09%> (+3.89%)
internal/extsvc/github/pulls.go	61.09% <86.66%> (+3.05%)
.../internal/codeintel/resolvers/graphql/locations.go	81.44% <0.00%> (-4.13%)
client/web/src/search/results/SearchResults.tsx	58.13% <0.00%> (+1.55%)
... and 202 more