fix /accounts/X/sub_accounts with more than 100 subaccounts

test plan:
 0. have an account with more than 100 subaccounts.
    I suggest creating these in the console:
    100.times { |x| account.sub_accounts.create! name: "subaccount #{x}" }
 1. go to /accounts/X/sub_accounts
 2. turn on web developer tools
 3. click the down arrow to the right of the account name to expand
    the list of accounts. confirm in the developer tools that the
    resulting ajax request is a reasonable size (only a few kilobytes,
    definitely not tens of megabytes)
 4. click the + icon next to a sub-account to create a sub-sub-account.
    this should work.
 5. click the down arrow next to the newly created sub-sub-account.
    you should not get an ajax error. (the list is blank.)
 6. refresh the page. you should be able to expand the main account
    and the sub-account to get to the sub-sub-account you created
    in step 4.

fixes CNVS-15950

Change-Id: I70a3bdd1f7186125c092a0107879863001367159
Reviewed-on: https://gerrit.instructure.com/42120
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>