Resolve "Create a service method to fetch the comments by one user"

Summary

This MR adds functionality for retrieving the following information from the GitLab REST API:

• the comments (notes) that a user has made on the root of an issue / merge request within a certain time frame
• the discussion threads (on both issues and MRs) that a user has commented on within a certain time frame For more details on the distinction please see the GitLab documentation. Note that discussion threads can only be retrieved per project. Thus, the number of API calls made to retrieve discussions per group = #api calls per project * #projects in a group.

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

The base functionality is implemented in the following methods:

• ProjectStatisticsService: getUserCommentsPerProject() and getDiscussionsPerUserProjectAndTimestamp()
• GroupStatisticsService: getUserCommentsPerGroup() and getDiscussionsPerUserGroupAndTimestamp()

Currently the UI does not reflect this functionality.

Issues and tasks closed

Issue #82 was split into issues #84 and #85 for granularity.

Relates to #82

Closes #84