RCE: Fix Pretty HTML Editor
RCE: Fix Pretty HTML Editor

Fixed typing html characters like `<` into the pretty
html editor.

flag=none
fixes MAT-208

test plan:
- In the new RCE
- Add some content to the wysiwyg view
- Switch to Pretty HTML Editor
- Type some html like `<p>hi</p>`
- Switch to raw html editor
- Type some more html
- Switch to wysiwyg view
- All the text should be there
- Switch back and forth between html editors and make sure
  it all looks good

Change-Id: If3d8dfb9b318fcac5eaa6f783e00e92a636e928a
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/265804
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>
QA-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>
Product-Review: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>

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