[TODO] Implement backporting policy configuration

[webknjaz]

What I have in mind is having a config structure that would allow the repo to say that the current version is X and the policy for bugfixes is X-1, the policy for security fixes is X-2 (for example).

Then, it'd allow us to implement the flow where having a bug label would trigger backports to the previous version and having security would trigger backports to two previous versions. Alternatively, this could be made explicit with needs_bug_backport / needs_security_backport (via opt-in or opt-out).

The last bit needed is to allow the repo to specify what part of the version spec to consider a bump (whether for v2.1.1 X-1 is considered v1.0.0, or v2.0.0, or v2.1.0).

Depends on: #5
Depends on: #6