Merge branch 'sh-link-bitbucket-server-import-docs' into 'master'

Link Bitbucket Server from import index page

See merge request gitlab-org/gitlab-ce!22752

doc/user/project/import/index.md

 # Migrating projects to a GitLab instance
 
-1. [From Bitbucket.org](bitbucket.md)
+1. [From Bitbucket Cloud (aka bitbucket.org)](bitbucket.md)
+1. [From Bitbucket Server (aka Stash)](bitbucket_server.md)
 1. [From ClearCase](clearcase.md)
 1. [From CVS](cvs.md)
 1. [From FogBugz](fogbugz.md)