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

[ci skip]

Fix documentation for Github integration authorization callback url.

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.

See merge request !4111