Due to a Rails bug, fetching the application settings from Redis
may prevent the attribute methods from being loaded for the `ApplicationSetting`
model. More details here: https://github.com/rails/rails/issues/27348

There was also a secondary problem introduced by overriding these
association methods which caused all default visibility levels to be
set to `nil`. Before, the previous implementation allowed the string
"20" to be saved as an integer, while now a table lookup happens
before that. We fix this by enforcing the integer value in the
controller and default to PRIVATE.

Closes #29674