Resolve "contributor emails in contributors graphs is case sensitive"

## What does this MR do?

changes author email to a case insensitive comparison so emails like "Somebody@foo.com" and "somebody@foo.com" are treated as a single author in contribution graphs.

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

should be pretty simple

## Why was this MR needed?

see above description

## Screenshots (if relevant)

#### before:
![before](/uploads/873a073e8d31e1094da6f050ba3e3f51/before.png)

#### after:
![after](/uploads/b9e534755615994f3e58442141c17f86/after.png)


## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [ ] ~~Added for this feature/bug~~
  - [ ] ~~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 it does - 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 #19550

See merge request !8021