change existing lti deep link FF to RootAccount
change existing lti deep link FF to RootAccount

closes INTEROP-7058
flag=none

why:
* originally we thought SiteAdmin would be best, but testing this will
involve turning these on only for some partners at a time, and
RootAccount is the best fit for that

test plan
* `<root account>.enable_feature! <either of these features` should
fail before this is checked out
* after checking this out, the same code should work, and the
feature flag should be on for that account, and off for
other root accounts

Change-Id: I8abb8a47162e3673cfbffbef4573fdba0c3c98bc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/274932
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
QA-Review: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
Product-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

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