Implemented data functions for commit heatmap by sprint/team

[SanJSp]

Partly implements #25

Changes

This PR introduces 2 new data functionalities. Now it is possible to filter the commits by team members and furthermore the data can now be separated by sprints. The data looks as follows

Without sprint segmentation

```json [ { "label": "Sprint 0", "value": [ { "x": 0, "y": 0, "v": 0 }, ... { "x": 23, "y": 6, "v": 0 } ] }, { "label": "Sprint 1", "value": [...] }, {...}, {...}, { "label": "Sprint not within sprint", "value": [...] } ] ```

Without sprint segmentation

```json [ { label: "Sprint 0", value: [ { "x": 0, "y": 0, "v": 0 }, ... { "x": 16, "y": 4, "v": 47 }, ... ] } ] ```

Checklist before merge

Developer's responsibilities

• [x] Assign one or two developers
• [x] Change code if reviewer(s) has/have requested it
• [x] Pull request build has passed
• [x] tested locally (in at least chrome & firefox if frontend)

[SanJSp]

FYI: @chrisma because there is further refactoring done by @LucPrestin in #23 regarding the selection and displaying of the metrics, the visualization of the commit heatmap by sprint will be implemented by him