Add `/volunteer/heatmap` endpoint

[TimJentzsch]

Relevant issue: GrafeasGroup/buttercup#29, Closes #166

Description:

Adds a /volunteer/heatmap endpoint to get the data necessary to create a heatmap like this:

It counts how many transcriptions a user has done in each hour of each day of the week.

For the response, unfortunately Sunday is 1 and Saturday 7, blame Django for that.

Checklist:

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