migrate has_X context_external_tool columns to separate table

currently every extension type has its own boolean db column
while this isn't strictly a problem, it is a little sad, and
given the fact that we will be adding more extension types soon,
it's probably better to take care of this now

also i'm refactoring some of the methods in context_external_tool

test plan:
* external tool regressions

refs #CNVS-13815

Change-Id: I6caf7b0e3d0de054906e3530f085fbfa9e8e499f
Reviewed-on: https://gerrit.instructure.com/38923
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>