Allow "ci skip" to be in any case

## What does this MR do?

This MR makes the check for the [ci skip] tag in a commit message case insensitive and adds support for [skip ci].

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

I couldn't understand why some of my commits were being built, even though they contained "[CI Skip]".

## What are the relevant issue numbers?
N/A
## Screenshots (if relevant)
N/A
## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] Tests
  - [x] Added for this feature/bug
  - [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 !4785