Add active_volunteer_count to summary endpoint

[TimJentzsch]

Relevant issue: Closes #198.

Description:

This counts the number of volunteers who completed a submission in the last two weeks. This gives us a sense of how many volunteers are actually still transcribing.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [x] Tests (if applicable)
• [x] Success Criteria Met
• [x] Inline Documentation
• [ ] Wiki Documentation (if applicable)