Run all merge policies by default, improve policy setting UX

### Context

Currently all merge policies are opt in, and it's a bit hard to understand what each policy done, and if it should be added.
We want to simplify the whole experience, without removing any functionality for our users.

The goal is to have a much simpler ux:
- We will remove the `NoRequestedChanges` and `Standard` checks
- In the Configuration repo, we'll simplify the yaml to have just two properties controlling these: `MaestroMerge` and `IgnoredChecks`. All merge properties will run by default. `MergePrs` will tell Maestro weather or not to merge PRs when all checks are successful, `IgnoredChecks` will tell the `AllChecksSuccessful` policy which checks to skip.
- If a PR has all green checks, but merge prs is set to false, Maestro won't update it.

Most of these changes will be "front end" only, we don't want to change how these policies are stored in the DB or anything like that.
So we'll have to make changes in the Ingestor and tell Maestro to only approve PRs if the `All Checks Successful` check is present.

Also we have to figure out a good way to "transition" in the configuration repository