GitHub Importer - Find users based on their email address

What does this MR do?

Try to find GitLab users based on their Github email address when importing projects from GitHub. The returned email by the GitHub API is the user's publicly visible email address (or null if the user has not specified a public email address in their profile). This means that the user should have signed up into GitLab with the same GitHub email publicly address or linked it on the settings page.

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

No.

Why was this MR needed?

Without this the require every user to first sign in to GitLab, associate their account with GitHub, and then import the project(s).

Currently, every user that has contributed an Issue/Pull Request require to sign in to GitLab, associate their account with GitHub, otherwise, the Issue/Merge Request will be associated with the user that imported the project.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #15181 (closed)

/cc @arihantar @dblessing @xiaogang_gitlab @Nowaker @mohyedeen