escape column header text in new gradebook grid
escape column header text in new gradebook grid

fixes SEC-611

test plan:
 1. create an assignment with the following name:
    ><svg onload=alert(1)>
 2. create an assignment group with the following name:
    ><svg onload=alert(1)>
 3. create a custom column with the following title:
    ><svg onload=alert(1)>
 4. visit New Gradebook
 5. ensure the browser alert does not appear

Change-Id: I1461b426f72250044f0942968d3e0e48cddf98c7
Reviewed-on: https://gerrit.instructure.com/125262
Tested-by: Jenkins
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
QA-Review: Anju Reddy <851202533fb1f0b615e4547ba226c76996d060dd@instructure.com>
Product-Review: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

instructure/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms