relation-broken events may have the departing app's data, requiring special handling.

[ca-scribner]

For a relation-broken event, the departing app's data may still be in the data bag when we handle that event. This is discussed in more detail in discourse and github.

To avoid leaving dead links in the sidebar, get_sidebar_items must handle this, returning only the current applications and omitting the departing one (at least until the above issues reach resolution). This is currently done in the charm by inspecting the environment variables and inferring what data we should omit.