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