Fix link preview for old RCE content
Fix link preview for old RCE content

closes LS-2020
flag=none

test plan:
  - in the RCE, add a file link and enable auto-open of the inline preview
  - edit the HTML and remove the instructure_file_link class name from
    the link
  - save
  > expect the link's preview to auto-open
  > expect this to  work with the rce_better_file_previewing and
    _downloading flags on or off

Change-Id: I2c17561cd0f6d6ec534031187624f8fc6feed598
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/260632
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
QA-Review: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
Product-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>

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