Resolve "Extract dataset/sample/investigation viewers to config files"

Closes #678 (closed)

Moves dataset/sample/investigation viewer configuration to JSON files {dataset/sample/investigation}.viewer.config.json.

Editing these files in JSON without type checking can be error-prone so I added a validation step using json-schema.

Remotes and microfrontends are now only defined in these json files:

• remotes.config.json for mapping microfrontend name to url
• {dataset/sample/investigation}.viewer.config.json for mapping data to microfrontend name + component name

These can be replaced at runtime so anyone can add new microfrontends without any change to the code.

Includes !619 (merged) to have the tests working properly