RFE: automatically update build list of an update if a build is untagged from the side-tag

[mattiaverga]

Suppose a user creates a side-tag update with two builds, foo-1.0 and bar-1.0.
Later they build foo-1.1 in the side-tag: they login in Bodhi, edit the update by hitting the refresh build list button and the new nvr replaces the old one.

If, instead of replacing an existing build, they want to only remove a build from being pushed within the update, they need to manually untag foo-1.0 from the side-tag with Koji and then they again need to login in Bodhi and hit the refresh button (there's no way to manually edit the build list in Bodhi).

My proposal is to add a consumer that processes the buildsys.untag messages to look if any user (other than Bodhi or Autopen) untag a build from a side-tag associated to an existing pending update and remove the build from the update list (and also remove the pending and testing tags possibly associated to the build).
That will simplify the previous example case nr. 2. I don't want to propose to automatically update the build list (example case nr. 1), because building a package in a side-tag doesn't automatically imply that the user wants it to be added to the update. But I think that is a user removes a nvr from a side-tag they certainly wants the build to be removed from the update.

Does it makes sense?