canvadocs: configurable mime types

closes CNVS-12992

Test plan:
  * configure canvadocs with more mime  types (e.g.
      Setting.set('canvadoc_mime_types',
        (Canvadoc.mime_types + %w[application/rtf text/rtf text/plain]).to_json)
  * restart your server
  * upload some documents of the newly added types (rtf or txt)
  * they should now preview in canvadocs

Change-Id: Ic3a1cb907f4da39cc8510221caa8b794b9d7f9f1
Reviewed-on: https://gerrit.instructure.com/34697
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>