Merge branch 'mr_widget_remove_source_branch_checkbox' into 'master'

Mr widget remove source branch checkbox

Fixes #1858

Renamed custom `checkbox` css class for remove source branch checkbox because it was being overwritten on Safari by `checkbox` from bootstrap in `_forms.scss`

After rename, alignment looks corrected

See merge request !1386

app/assets/stylesheets/sections/merge_requests.scss

@@ -29,7 +29,7 @@
       line-height: 20px;
       font-weight: bold;
 
-      .checkbox {
+      .remove_source_checkbox {
         margin: 0;
       }
     }

app/views/projects/merge_requests/show/_mr_accept.html.haml

@@ -18,7 +18,7 @@
             = f.submit "Accept Merge Request", class: "btn btn-create accept_merge_request"
           - if can_remove_branch?(@merge_request.source_project, @merge_request.source_branch) && !@merge_request.for_fork?
             .accept-control
-              = label_tag :should_remove_source_branch, class: "checkbox" do
+              = label_tag :should_remove_source_branch, class: "remove_source_checkbox" do
                 = check_box_tag :should_remove_source_branch
                 Remove source-branch
           .accept-control