Add label requirement to dependency checker and make the checker being green mandatory

[dfawley]

Today when the dependency checker fails, meaning a new dependency appeared in an exported package, it shows a red X in the PR, but the checker itself is marked as optional, meaning the PR can be merged.

This relies on a human process that happens to notice the red X and investigates.

Proposal:

1. Make the dependency checker a mandatory check.
2. Create a new github label, e.g. "Dependency Changes Verified" or something similar.
3. Make the dependency checker consider its status green if the label appears.

We still need to solve the problem of:

1. PR is created with minor dep change.
2. Label is added.
3. PR is updated to add a major dep change that we don't want.

So this proposal is still incomplete.