a8n: Introduce an enterprise version of repo-updater

Created by: mrnugget

Last week @tsenart and I decided that the process that updates changesets from code hosts (i.e. GitHub pull requests) should live in repo-updater, since that is where we already have a lot of the code that interacts with code hosts and because repo-updater acts as a "singleton" (no replicas) which makes it easy to start a single, long-running goroutine.

But since A8N needs to live in ./enterprise and we cannot mix OSS and enterprise code in repo-updater, we took a shortcut last week before release and moved the code from repo-updater to ./enterprise/cmd/frontend.

Now, with more time available, we want to do the proper thing and create an enterprise version of repo-updater that can then include the A8N code.

This PR here is the first step. It copies what frontend and management-console do and creates a shared pkg for repo-updater. It also declares repo-updater to be an enterprise command that can be built from ./enterprise/cmd/repo-updater.

There's not a lot of code here (the first commit moves the old main() function to the new shared.Main()), it's mainly adjusting scripts.

My main question: did I miss anything? Since I've never done this before, hopefully one of you can tell me :)

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Random not-so-fun-but-enterprise fact: a8n enterprise could be shortened to a8n e8e

Created by: codecov[bot]

Codecov Report

Merging #5714 into master will increase coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5714      +/-   ##
==========================================
+ Coverage   39.27%   39.27%   +<.01%     
==========================================
  Files        1145     1145              
  Lines       55754    55748       -6     
  Branches     5609     5608       -1     
==========================================
- Hits        21895    21893       -2     
+ Misses      31797    31793       -4     
  Partials     2062     2062

Impacted Files	Coverage Δ
web/src/repo/commits/RepositoryCommitsPage.tsx	0% <0%> (ø)
web/src/search/results/SearchResultsInfoBar.tsx	66.66% <0%> (ø)
web/src/savedSearches/SavedSearchListPage.tsx	0% <0%> (ø)
shared/src/e2e/driver.ts	0% <0%> (ø)
web/src/repo/commits/GitCommitNodeByline.tsx	100% <0%> (ø)
web/src/repo/GitRef.tsx	0% <0%> (ø)
cmd/frontend/graphqlbackend/repository.go	22.81% <0%> (+0.97%)

Created by: mrnugget

How do you plan on including the enterprise bits?

My current idea is to add something like a callback to shared.Main(), so you can do something like this:

shared.Main(func (db *sql.DB, syncer *repos.Syncer) {
    a8nSyncer := NewA8NSyncer(db)
    go a8nSyncer.Run()
})

Open for suggestions though, since I haven't explored possible solution a lot yet