Use custom JSON-LD context

[acka47]

From #432 (comment):

Create and use a project-specific JSON-LD context (for now, I have added "@context": "https://schema.org" for most entries which is enough for development. In the end, we will have to take a deeper look and create a application-specific context.)

One concrete thing to address is this:

Currently, we use JSON-LD language maps for name and description. To make this valid JSON-LD we will have to adjust the context accordingly:

{
  "name": {
    "@container": "@language"
  },
  "description": {
    "@container": "@language"
  }
}