update outcome "new" buttons from icon-add to icon-plus
update outcome "new" buttons from icon-add to icon-plus

Fixes: CNVS-29688

test plan:
- as an admin
- navigate to a course
- select "outcomes" from the sub-nav
- the "Outcome" and "Group" buttons should now have the
  plus icon vs the circle-plus (icon-add) and the
  wording "new" removed
  example of new view: http://screencast.com/t/eldwG4i5Id2

Change-Id: Ie43630b8e7ba51fa9fd99dbc4198385818e9112a
Reviewed-on: https://gerrit.instructure.com/81223
Reviewed-by: Chris Hart <b82b40b84a2f06957ec9caf1768340bd6b929cc4@instructure.com>
Product-Review: Blake Simkins <b120bacfd01287a1de7e8077d020b48e9650385b@instructure.com>
Tested-by: Jenkins
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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