Tracking contributions

I have a goal of accurately crediting all contributions to ActivityWatch, to motivate both new and existing contributors.

The work done so far has mostly been on the 'Contributors' page on the website: https://activitywatch.net/contributors/

But recently, I've also updated the changelogs to explicitly list the GitHub usernames of authors who have contributed to a particular release (like for v0.12.0: https://github.com/ActivityWatch/activitywatch/releases/tag/v0.12.0)

## Contributions currently credited

- [x] Commits/PRs
- Could be split into feat/fix/docs/typo
- [ ] Issue comments
- Could count reacts, like how many thumbs-up/hearts a user has received for their comments
- [ ] Discord activity
- We currently have a "XP system" using the MEE6 bot, but we could do better

## Ideas

- Would be cool to add a timeline for each user with a span between their first and last activity, to visualize when they first contributed, and their activity over time.