Custom request prompt defaults to `nil`

[orenyk]

This causes 500 errors when a new request is created until the setting is populated with any string. This must be done by an admin visiting the Settings page and simply submitting the form with no changes. We should modify the migration to prevent nils / specify the default of an empty string and also validate for this in the model. I think this can safely be resolved in master only.

[orenyk]

At @caseywatts's suggestion, we're going to add a new migration that sets sane defaults for many of our app config parameters. This will ensure that a new installation works out of the box an doesn't throw any NoMethodErrors.