COOPS-UI: revisit mixin usage re: JSON validator package

## Description:
Using the 'official' decorators, TypeScript components cannot import classic JS mixins (eg, they are not installed / do not run correctly). However, I found this package which may solve this problem: https://github.com/ktsn/vue-typed-mixins

Specifically, it may be possible to import and use the json-schema-validator package using the above mechamism. See ticket #852. If this works then we would gain the ability to import a JSON schema file instead of specifying the schema in code. (It will still be an issue if, say, registered office address requires different validation from director address. It may also still not be possible to resolve referenced schemas.)