Size equations to surrounding text
Size equations to surrounding text

closes LS-2845
flag=scale_equation_images

test plan:
  - turn on site admin flag scale_equation_images
  - in the rce, create some content with a large font
  - with the text caret in the large font area (you can tell by
    looking at the blinking cursor) insert a math equation
  > expect the equation to be scaled to match the surrounding font size
  - click on the equation image and change its font size using the rce toolbar
  - edit equation and save. you don't have to change anything
  > expect the equation to be in the new font size
  (some day we'll detect the change in font size and automagically fix it)

Change-Id: I323bf4162521cf88d2a993b8e62ef697114f8959
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/278299
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
QA-Review: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
Product-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms