Allow importing from Github using Personal Access Tokens

Originally opened at !4005 (closed) by @ericidema.

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What does this MR do?

• Made changes to Gitlab::GithubImport::Client so that it can be used with Github Personal Access Tokens without the need for OAuth.
• Added UI to collect Personal Access Token from user.
• Detect if the user has logged in with GitHub and use OAuth to skip the Personal Access Token form.

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

Twin Omnibus MR: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/774

What are the relevant issue numbers?

Closes #13970 (closed).

Screenshots

GitHub import is configured

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GitHub import is not configured

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Does this MR meet the acceptance criteria?

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