Hide Composite Identity Service Accounts without associated triggers

Problem

Both for custom and foundational flows service accounts are created and then added to projects. These accounts, by default behave like any other account, which means they show up in Assignee dropdowns or @ mention lists to be picked.

However, many flows will not have actual triggers associated with an assignment or mention happening, since they are created for such a case. E.g. the service account for a code-review flow will not have triggers for being assigned to issues, similarly the service account for an issue triage flow would not have the triggers for being assigned a review.

Still showing these account in situations where they do not "trigger" anything can be confusing to users and distract from accounts that do trigger something.

Proposed solution

Only show composite identity service accounts in situations where they have an associated trigger.

Additional information

#580296 (closed) also touches how these Service Accounts are handled.