Cache inexistence of the GitHub username when importing a repo to GitLab

What does this MR do and why?

It adds the rescue Octokit::NotFound exception inside the UserFinder and also cache the inexistence of the user to not make several requests to the GitHub API fetching the same information about the user. It also removes rescuing this exception in the GitHub import importers:

• Gitlab::GithubImport::Importer::PullRequestReviewImporter
• Gitlab::GithubImport::Importer::PullRequestMergedByImporter

It implements #385777 (closed).

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