fix: do not consider the overview cache multiple times in a releasetrain

[sven-urbanski-freiheit-com]

This is a performance improvement that solves 2 issues with the overview cache:

1. It reduces the amount of times we write the overview in a release train from n=number of apps to 1 per env. The 1 write contains multiple changed apps.
2. When creating a new app or when undeploying, it does not force a recalculation of the whole overview, but instead only updates the app.

Further cases that also force a complete recalculation of the overview cache (like creating a new environment) are not handled in this PR.

Note that some functions like CalculateWarnings had to be moved to another module but were not changed.

Ref: SRX-4WQBET