Do not pass project.owner because it may return a group and Labels::FindOrCreateService
throws an error in this case.
Fixes #23694.

Closes #22998

If the GitHub project repository has wiki, we should not create
the default wiki. Otherwise the GitHub importer will fail because
the wiki repository already exist.

This bug was introduced here
https://gitlab.com/gitlab-org/gitlab-ce/commit/892dea67717c0efbd6a28f763
9f34535ec0a8747

* No need to re-fetch issues from GH to read their labels, the labels
  are already there from the index request.
* No need to look up labels on the database for every application, so we
  cache them.

This should avoid having large memory growth when importing GitHub repos
with lots of resources.

- Fixed and added specs.
- Added different namespace options depending on user privilages
- Updated docs.

# Conflicts:
#	lib/gitlab/github_import/importer.rb

This stands as an alternative to using OAuth to access a user's Github
repositories.  This is setup in such a way that it can be used without OAuth
configuration.

From a UI perspective, the how to import modal has been replaced by a full
page, which includes a form for posting a personal access token back to the
Import::GithubController.

If the user has logged in via GitHub, skip the Personal Access Token and go
directly to Github for an access token via OAuth.