Implemented different unit for closed pull requests

[SanJSp]

Fixes #8

Changes

This PR changes the unit of the displayed closed pull request times from milliseconds to hours, rounded to the second digit. At first, I'd loved to implement this in DD HH:MM:SS, but this was not possible to be displayed in the chart. Therefore, now only the hours.

Screenshots

Before

After

Checklist before merge

Developer's responsibilities

• [x] Assign one or two developers
• [ ] 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]

According to Christophs comment, I renamed the metric to Pull request open duration