24 issue submit time

[LucPrestin]

Closes #24

Changes

• adds a new data function to extract issue submit times from the github api
• applied the resulting data to a heatmap

Screenshots

Checklist before merge

Developer's responsibilities

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

[LucPrestin]

As you suspected, I introduced the git_artifact_name only for the labels when hovering over the data