fix expected speedgrader url template format

rails 2.3 to 3.2 changed the way that anchor tags are escaped in their url
builder methods from using CGI.escape (in 2.3) to URI.escape with a custom safe
list (in 3.2).  The changed caused the colon to stop being escaped.

fixes CNVS-14890

test plan:
- create an assignment discussion, and post to it as a several students
- as a teacher, use the "Open in Speedgrader" link in the discussion post cog
  menu for each student.
- it should open speedgrader to that specific student

Change-Id: I3d5aaae33b9b0e4914e09fa53292d9e042a91f26
Reviewed-on: https://gerrit.instructure.com/39752
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>