Fix documentation for Github integration authorization callback url.
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.

martijnvermaat/gitlabhq,daiyu/gitlab-zh,LUMC/gitlabhq,darkrasid/gitlabhq,icedwater/gitlabhq,iiet/iiet-git,darkrasid/gitlabhq,t-zuehlsdorff/gitlabhq,martijnvermaat/gitlabhq,mmkassem/gitlabhq,allysonbarros/gitlabhq,shinexiao/gitlabhq,htve/GitlabForChinese,darkrasid/gitlabhq,dreampet/gitlab,t-zuehlsdorff/gitlabhq,SVArago/gitlabhq,stoplightio/gitlabhq,LUMC/gitlabhq,screenpages/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,allysonbarros/gitlabhq,openwide-java/gitlabhq,t-zuehlsdorff/gitlabhq,jirutka/gitlabhq,darkrasid/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,dplarson/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,htve/GitlabForChinese,axilleas/gitlabhq,stoplightio/gitlabhq,allysonbarros/gitlabhq,dplarson/gitlabhq,screenpages/gitlabhq,iiet/iiet-git,openwide-java/gitlabhq,screenpages/gitlabhq,shinexiao/gitlabhq,martijnvermaat/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,SVArago/gitlabhq,SVArago/gitlabhq,htve/GitlabForChinese,SVArago/gitlabhq,mr-dxdy/gitlabhq,shinexiao/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,icedwater/gitlabhq,mr-dxdy/gitlabhq,axilleas/gitlabhq,martijnvermaat/gitlabhq,daiyu/gitlab-zh,mr-dxdy/gitlabhq,screenpages/gitlabhq,shinexiao/gitlabhq,LUMC/gitlabhq,icedwater/gitlabhq,allysonbarros/gitlabhq,openwide-java/gitlabhq,htve/GitlabForChinese,daiyu/gitlab-zh,icedwater/gitlabhq,LUMC/gitlabhq,mr-dxdy/gitlabhq,daiyu/gitlab-zh,iiet/iiet-git,openwide-java/gitlabhq,jirutka/gitlabhq