Allow importing from Github using Personal Access Tokens

_Originally opened at !4005 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.

## Screenshots

### GitHub import is configured

![github_import_configured](/uploads/151e4f0edf3f87bfa03c2d97dda8b3d8/github_import_configured.png)

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### GitHub import is not configured

![github_import_not_configured](/uploads/cb129f7e2ffe66cceb28ccd9af480284/github_import_not_configured.png)

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## 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)
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] 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 !4938