Add padding to Bootstrap input add-on element

Fixes: CNVS-17189

Test plan:
- Go to any course
- Open Assignments
- Click on the cog in the top-right
- Check "Weight final grade based on assignment groups"
- Make sure the input looks like this:
  http://cl.ly/image/2Z0C3y3v3N3r/add-on-fix.png
  ...and NOT like this:
  https://www.dropbox.com/s/76dv04foxzudgvc/Screenshot%202014-12-02%2016.15.07.png?dl=0

Change-Id: Id509951bc7e294d8f968ad4d3e17d462cbc19fc9
Reviewed-on: https://gerrit.instructure.com/45198
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>