Take care of in-flight side tag updates after Bodhi activation point

[nphilipp]

Story

As a user of Bodhi who submitted an update from a side-tag before the Bodhi activation point of its release,

I want that the builds in the update get retagged into the pending-signing or pending-testing tag of the release, and that the side tag and its auxiliary tags are deleted after the Bodhi activation point of the release,

so the update continues to progress as a side-tag update from a branched/stable release.

Background

For updates from a side-tag in a stable release, we'll transfer builds to normal tags on submission (#3800), so that they don't get stalled either. Let's do the same here, maybe in a background task as soon as the composed_by_bodhi flag is set for the release.

Acceptance Criteria

• When the composed_by_bodhi flag of a pending release is set through the API, a backend task is created which "re-routes" the update and its builds into the workflow of a branched/stable release.
• This task re-tags the updates/builds into the pending-signing or pending-testing tag of the release (depending on if they were tagged with the pending-signing or pending-testing tag of the side-tag) and removes the side-tag and its auxiliary tags.
• Tests exist for the new functionality.