Reduce contributions calendar data payload
## What does this MR do?
Reduces the contributions calendar data payload by only passing contribution dates and counts
## Are there points in the code the reviewer needs to double check?
Please double check my javascript code 😃
## Why was this MR needed?
Reduce user's bandwidth and decrease website loading time
## What are the relevant issue numbers?
Closes #20862
## Screenshots (if relevant)
Before:
```
new Calendar(
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'/u/root/calendar_activities'
);
```
After:
```
new Calendar(
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'/u/root/calendar_activities'
);
```
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
- [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
See merge request !5784
