Fix contributions calendar month label truncation

## What does this MR do?
Expands the width of the contributions calendar by 3px when the last column is a new month to prevent the month label from being truncated

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
Fixes an existing UI issue

## What are the relevant issue numbers?
Closes #20844 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-11_at_1.24.47_PM](/uploads/682a616a335fffd610342037c41ab9ad/Screen_Shot_2016-08-11_at_1.24.47_PM.png)

After:
![Screen_Shot_2016-08-11_at_1.24.13_PM](/uploads/6e7aa12dd9e3a2c570f1327aaef09fc7/Screen_Shot_2016-08-11_at_1.24.13_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 [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 !5779