Merge branch 'ff-merge' into 'master'

Fast-forward merge button

For #337, #338 

- [x] fast-forward merge option in project settings
- [x] fast-forward merge button
- [x] message if ff is not possible
- [x] rebase button if ff is not possible and rebase is enabled
- [x] rebase button depends on ff-only setting
- [x] remove `rebase enabled by default` checkbox from project settings
- [x] implement rebase feature
- [x] write tests
- [x] update CHANGELOG and documentation
- [x] mention in 8.0 blog post

See merge request !485

app/views/projects/merge_requests/widget/open/_ff_accept.html.haml

@@ -19,7 +19,7 @@
         - else
           .accept-action
             = f.button class: "btn btn-reopen rebase-mr" do
-              Rebase from #{@merge_request.target_branch}
+              Rebase onto #{@merge_request.target_branch}
       .accept-control
         Fast-forward merge is not possible. Branch must be rebased first
 
@@ -32,4 +32,4 @@
   $('.rebase-mr-form').on 'ajax:before', ->
     btn = $('.rebase-mr')
     btn.disable()
-    btn.html("<i class='fa fa-spinner fa-spin'></i> Rebase in progress. It could take some time")
+    btn.html("<i class='fa fa-spinner fa-spin'></i> Rebase in progress. It could take some time")
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