bump up pagination for feature flags to 20 instead of 10

It seems like eventually we'll want to update the front-end backbone code to
do proper pagination on this, but we can certainly handle 20 feature flags
before doing this.

fixes CNVS-13461

test plan:
  - as an admin
  - visit the account settings page
  - click on the feature flags tab
  - all 11 feature flags should show up including draft state at the bottom
  - try turning the feature flags on/off to make sure things still work fine

Change-Id: Ifc7c31a48159f77d63a846c472567ae88f913f76
Reviewed-on: https://gerrit.instructure.com/35967
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>