Permalink Validation

[ErisDS]

This is mostly done already, but there are a few improvements / updates needed in order to ensure that permalinks saved to the database are definitely valid.

Validation of settings is handled by a special validation function which uses the configuration in default-settings.json to know how to validate each individual setting.

As you can see here there are already some validation rules for permalinks, but they are not complete.

The full spec for what constitutes a valid permalink can be found in #1631. Most notably, the :author url part is missing, there's no affordance for custom strings, and there is no assurance that at least one of :slug or :id is present.

As well as updating the rules, it would be great to see some tests added which verify that the validation is not permitting invalid permalink configs to be stored.

[ErisDS]

Closing this for now with the later tag, we ca pick this back up if we decide to add a permalinks UI, or if this becomes a problem for channels at some point.