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.