Remove tabstop from the WIP toggle links

[ci skip]

app/views/shared/issuable/_form.html.haml

@@ -14,7 +14,7 @@
     - if issuable.is_a?(MergeRequest)
       %p.help-block
         .js-wip-explanation
-          %a.js-toggle-wip{href: ""}
+          %a.js-toggle-wip{href: "", tabindex: -1}
             Remove the
             %code WIP:
             prefix from the title
@@ -22,7 +22,7 @@
           %strong Work In Progress
           merge request to be merged when it's ready.
         .js-no-wip-explanation
-          %a.js-toggle-wip{href: ""}
+          %a.js-toggle-wip{href: "", tabindex: -1}
             Start the title with
             %code WIP:
           to prevent a