Fix escaped angle bracket's in integration documentation.

There are several escaped angle brackets in our integration docs.  While these
render fine within GitLab, they are broken when rendered on doc.gitlab.com because
pandoc does not escape them correctly.

You can see the problem here: http://doc.gitlab.com/ce/integration/github.html

Note that the strings `<Organization>` and `<Your Name>` are being interpreted as html tags.

It looks like doc.gitlab.com is using pandoc: https://gitlab.com/gitlab-com/doc-gitlab-com/blob/master/generate.rb#L64

You can verify these changes by running something like this and then inspecting `test.html`:

    pandoc --from markdown_github-hard_line_breaks -o test.html doc/integration/twitter.md

You can also verify that GitLab continues to render the docs correctly by checking, for example, http://localhost:3000/help/integration/twitter.md


See merge request !4128

There are several escaped angle brackets in our integration docs.  While these
render fine within GitLab, they are broken rendered on doc.gitlab.com because
pandoc does not escape them correctly.

There are two callbacks that could be used with Github integration:

  * /import/github/callback (used by project import)
  * /users/auth/github/callback (used by OmniAuth)

Github's documentation suggests that authorization callback url should be
set to the longest common path.

https://developer.github.com/v3/oauth/#redirect-urls

Configuring according to the previous documentation resulted in a
redirect_uri_mismatch error from Github when logging in via OmniAuth.

These changes were pulled from GitLab EE to support configuring
an alternative API URL than the default https://api.github.com.
In addition, the `verify_ssl` flag allows users to disable SSL cert
checking.

One modification: add a default `args` option if it does not exist
to avoid breaking existing configurations.

Update github.md

[ci skip]