hide new conversations ui
hide new conversations ui

test plan:
  * navigate to conversations as a site admin and verify
    that you can still switch between the old/new
    interfaces;
  * navigate to conversations as a user in another role and
    verify that you cannot switch to the new ui.

Change-Id: Id1f629958f5870a8700310419959722c361ce3ce
Reviewed-on: https://gerrit.instructure.com/24355
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jon Willesen <eb5ce5e6a76c243326fecfa94534101600f87bd9@instructure.com>
Product-Review: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>

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