Implement UI display and burndown chart calculation for GitHub draft mode

Created by: eseliger

Next PR in the series towards https://github.com/sourcegraph/sourcegraph/issues/7998.

Today at "Let's add support for draft PRs"::

• Extends the ExternalChangesetState by DRAFT (plus UI representation)
• Adds drafts to the calculation of historic changeset data
• Exposes the number of drafts on the changesets.stats and on the changesetCountsOverTime fields

Still to be figured out: Do webhooks need a special treatment or will they just work? (Maybe there will be pointers in the reviews.. ) Otherwise I'll give it a shot tomorrow. Can be reviewed commit-by-commit.

Created by: codecov[bot]

Codecov Report

Merging #14742 into main will decrease coverage by 0.01%. The diff coverage is 50.66%.

@@            Coverage Diff             @@
##             main   #14742      +/-   ##
==========================================
- Coverage   52.25%   52.23%   -0.02%     
==========================================
  Files        1554     1554              
  Lines       79295    79340      +45     
  Branches     6977     6994      +17     
==========================================
+ Hits        41433    41444      +11     
- Misses      34119    34153      +34     
  Partials     3743     3743              

Flag	Coverage Δ
#go	52.44% <46.96%> (-0.04%)
#integration	30.56% <22.22%> (+0.01%)
#storybook	22.20% <77.77%> (+0.03%)
#typescript	51.72% <77.77%> (+0.02%)
#unit	33.60% <22.22%> (-0.01%)

Impacted Files	Coverage Δ
.../src/enterprise/campaigns/detail/BurndownChart.tsx	92.59% <ø> (ø)
.../enterprise/campaigns/detail/CampaignStatsCard.tsx	100.00% <ø> (ø)
...ent/web/src/enterprise/campaigns/detail/backend.ts	60.00% <ø> (ø)
...campaigns/detail/changesets/ChangesetFilterRow.tsx	55.73% <0.00%> (-1.89%)
cmd/frontend/graphqlbackend/campaigns.go	0.00% <ø> (ø)
...terprise/internal/campaigns/resolvers/changeset.go	69.15% <0.00%> (ø)
...e/internal/campaigns/resolvers/changeset_counts.go	88.88% <0.00%> (-11.12%)
internal/campaigns/changeset.go	49.45% <0.00%> (-5.21%)
internal/campaigns/changeset_event.go	0.00% <0.00%> (ø)
...ternal/campaigns/resolvers/changeset_connection.go	73.80% <33.33%> (-1.50%)
... and 10 more