Purge resolved issues should check if issue contexts have been orphaned

Currently, there are edge cases where the continuous issue tracking just doesn't work (e.g. Apros #14484-2).

Useless issues are left hanging in the issue list, with their contexts invalidated but the issues still exist in the database issues VG. No action on the UI can currently remove invalid issues coming from "continuous" sources. Since continuous issue sources should and also does reproduce any issues that are still valid, if one was to just purge all issues.

The proposal is to:

• Remove also continuous-source issues whose contexts can be considered disconnected/orphaned according to how #295 (closed) was implemented
  • This should make it possible to get more rubbish issues removed from the issue list with Purge Issues.