Refactor/clean-up assessment vs. issue vs. submission

[dgor82]

See the one or two commits early in San Hejmo when this was introduced and clean up.

• [ ] Introduce Submission.
• [ ] Deduplicate with Issue.
• [ ] See IIssueFactory<T> in Interfaces. It says the Generic T is never used. Can I remove it? Apply to Submission or Assessment factory likewise...
• [ ] Clean up on all levels, from Workflows to Model