Open SpeedGrader links in To Do list in a new tab

closes LS-2566
flag=none

test plan:
- create an assignment as a teacher
- submit the assignment as a student
- as the teacher, observe the "Grade assignment" To Do item in the To Do
  list on the course sidebar (and on the Dashboard)
- in both To Do list locations, verify that clicking on the link opens
  SpeedGrader in a new tab

Change-Id: I2ca0d578cc24be7cd783a3b4d0e2326cfc632553
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/272532
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Eric Saupe <eric.saupe@instructure.com>
QA-Review: Eric Saupe <eric.saupe@instructure.com>
Product-Review: Peyton Craighill <pcraighill@instructure.com>

app/views/courses/_to_do_list.html.erb

@@ -41,6 +41,7 @@ show_legacy_todo_list ||= false
           <a
             class="item"
             href="<%= assignment.gradebook_path %>"
+            target="_blank"
             data-track-category="dashboard"
             data-track-label="todo needs grading"
           >

ui/features/k5_dashboard/react/Todo.js

@@ -87,6 +87,7 @@ const Todo = ({assignment, context_name, html_url, ignore, needs_grading_count,
       <Flex as="div" direction="column" margin="0 small 0 0" width="27rem">
         <Link
           href={html_url}
+          target="_blank"
           isWithinText={false}
           theme={{
             fontWeight: '700'