Replace contributions calendar timezone payload with dates

## What does this MR do?
Fixes a ~regression bug introduced in !5784 whereby the calendar squares would not render due to a timezone mismatch between the server timezone and the browser (user) timezone. I am now returning the dates instead of the timezone from the backend. There's a good chance we will still have a calendar off by one error (which is an existing issue #1943) but passing dates should make sure that the calendar activity squares render correctly across timezones.

## Are there points in the code the reviewer needs to double check?
Double check that this displays the correct calendar squares when the GitLab instance is hosted in a different timezone than the user viewing it

## Why was this MR needed?
Fixes ~regression issue 😞 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-09-13_at_9.47.06_PM](/uploads/d8b41b9b43320f021452e39f29b0e194/Screen_Shot_2016-09-13_at_9.47.06_PM.png)

After:
![Screen_Shot_2016-09-13_at_9.46.11_PM](/uploads/e09a3b35eef77ee1cf73b924c3815d3a/Screen_Shot_2016-09-13_at_9.46.11_PM.png)

## 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 [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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)

## What are the relevant issue numbers?
Closes #21921

See merge request !6336