Add `utc_offset` query parameters to `submission/rate` and `submission/heatmap` endpoints

[TimJentzsch]

Relevant issue: Closes #301

Description:

This allows the client to specify the UTC offset in seconds, which can be used to localize the results. I tried to do this client-side on Buttercup, but for the rate we don't have enough info to do it and for heatmap it doesn't work for time zones like +01:30. So moving this functionality to the server side makes sense for us (and might be beneficial for third-party clients or our other bots).

Checklist:

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