Conversations inbox opt out

Closes CNVS-11854

Test Plan:
1) Restart  your server after checking out this code (there is new feature flag)
2) As a user, visit /profile/settings and see that there is no option
   for disabling the inbox
3) As an instructure employee viewing settings for the root account,
   turn "on" the inbox opt out feature flag. If the button says "allow"
   instead of "on", you are in the wrong place and want to be on the
   account's settings, not the site admin settings.
4) As a user, visit your settings again: /profile/settings
5) See that you can now disable the inbox.
6) Do so.
7) Check the that conversations notification settings no longer appear
   at /profile/communication
8) Check that when you have new conversation messages, the unread
   messages badging doesn't display on the inbox link in the main nav
9) Visit your settings a 3rd time, and un-disable the inbox
10) check that the inverse of 7 & 8 are now true

Change-Id: I061e2d3ab9052a7809fb43f80d8e2e92d99316ee
Reviewed-on: https://gerrit.instructure.com/38313
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: David Josse <david@instructure.com>