CODEOWNERS Syntax Check is Case Sensitive, Approval Rules are not

Summary

When adding users to CODEOWNERS using their Usernames (e.g. @johndoe) while the Username is in fact @JohnDoe the file displays the following error messege when viewed in the UI:

@JohnDoe can still approve MRs just fine.

Steps to reproduce

1. Create CODEOWNERS file
2. Add user with "wrong" captialization.
3. View CODEOWNERS file in code view.

Example Project

https://gitlab.com/florianknecht/example-codeowners-casesensitive

What is the current bug behavior?

Codeowners file view is displaying a syntax error.

MR Approvals work fine.

What is the expected correct behavior?

Codeowners file view is not displaying any error.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

 (For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`)  (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)  

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes

Patch release information for backports

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High-severity bug remediation

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