Draft: Resolve "Create a service method to fetch the comments on MRs / Issues"

Summary

This MR adds four new service methods:

• two for retrieving all comments on all issues in one project / group within a certain time stamp
• two for retrieving all comments on all merge requests in one project / group within a certain time stamp

The methods take into account (only) the time stamp of the comment creation and filter Notes accordingly.

A new utility class is also added that allows for filtering statistics retrieved from GitLab. In this instance, it is used to filter retrieved Notes by username and time stamps.

Checklist

All items on this checklist should be checked.

• I have added tests for the functionality that I added.
• I adhere to the style guide.

How to review

Changes are implemented in the files:

• GroupStatisticsService.java - methods getCreatedCommentsOnGroupMRs() and getCreatedCommentsOnGroupIssues()
• ProjectStatisticsService.java - methods getCreatedCommentsOnProjectMRs() and getCreatedCommentsOnProjectIssues()

Currently the UI does not reflect these changes.

Issues and tasks closed

Closes #81