Create Dashboard with PR-Level Review & Comment Tracking, Filtering, and Metrics

**Is your feature request related to a problem? Please describe.**

Reviews and comments play a crucial role in project development. To better track contributions, we should include them as a metric.

After further discussion from #447 , it is agreed that the user should have a PR-level view of each PR within the repo. Hence, the design of the dashboard has been improved to show this.

**Describe the solution you'd like**
@NorbertLoh and I have redesigned the dashboard as such:


The features of the dashboard is as follows:
- shows the **timeline of reviews/comments per PR**, and who is responding to who during the PR lifetime
- team members are represented by the colored circles, each one represents a comment or PR within the timeline
- shows the **sum of reviews/comments contributed per member, per PR**
- tallies up the total at the end of the table to give a repo-level view of reviews/comments made per team member

Users can also choose to filter this table based on a few conditions. Some possible filters in mind are:
- Creator of PR (filters PR rows based on creator)
- Time frame (range filter, user can specify start and end time for timeframe)
- Milestones

**Additional context**
This may be a rather complex feature, if there is anything I can clarify further in my explanation feel free to let me or @NorbertLoh know! Any other suggestions will also be greatly appreciated. Thanks!

EDIT:
For now, @NorbertLoh and I intend to work on this feature away from `main` branch, and hence will make our PRs to `reviews-dashboard` branch instead. We will create a PR to merge the full feature to `main` once it is completed.

Start by reviewing the discussion in issue #447 and the redesigned dashboard image to understand the intended PR-level view. Work from the reviews-dashboard branch, where the authors intend to develop the feature. Done means showing review and comment timelines, per-member and repository totals, and filters for PR creator, time frame, and milestones.

Written by the indexing model from the issue text.