Advertise the PAT as an alternative unless GH import is not configured.
Signed-off-by: NRémy Coutable <remy@rymai.me>

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From a UI perspective, the how to import modal has been replaced by a full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.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

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Links like (doc/web_hooks/web_hooks.md) work in the GitLab source code
web interface, but the HTML generator produces broken links in the `doc`
subdirectories:
http://doc.gitlab.com/ce/hooks/doc/web_hooks/web_hooks.html
instead of the right one
http://doc.gitlab.com/ce/web_hooks/web_hooks.html
in
http://doc.gitlab.com/ce/hooks/custom_hooks.html.

Fixes #14338

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Signed-off-by: NAnatoly Borodin <anatoly.borodin@gmail.com>

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See https://gitlab.com/gitlab-org/gitlab-ce/commit/4a203aab57c69ea402939cf8fc423a0e5aaa1cab