Add `secret` column to Settings
Add `secret` column to Settings

In a subsequent commit, this will allow a setting to be marked as
"secret", and its value redacted in the forthcoming Settings UI.

refs DE-1138
flag=none

test plan:
- verify the column is added, and `setting=` is successful

Change-Id: I0dfad0ea584e199828ff04fabe91964069d3e676
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/289857
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Migration-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Isaac Moore <258108225390078ef47d1319acab49ad7948abf1@instructure.com>
Product-Review: Isaac Moore <258108225390078ef47d1319acab49ad7948abf1@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms