Create helper script to clean up tagged builds in ELN

[sgallagher]

This is currently a manual process, but we should design a script that will compare the current contents of the tag(s) with what is expected from Content Resolver and present the set of packages that should be untagged to the user for evaluation.

There should also be a companion script (or alternate execution method of the same script) that applies those changes when the human has approved.

(Suggestion: One script that outputs a JSON file as well as a human-readable format and another that can accept the JSON file and apply the changes.)

[yselkowitz]

See context in fedora-eln/elnbuildsync#67 but this is not something that EBS has the context to handle safely.