Add endpoints for meta models

[magicknot]

List meta models

• GET /metamodels returns a list of all meta models available.

List a single meta model

• GET /metamodels/:id returns the meta model identified by id.

Create a new meta model

• POST /metamodels creates a new meta model. A JSON body must be passed:

{
  "name": "",
  "description": "",
  "entity_types: [],
}

Assign a meta model to a document

• PUT /documents/:id adds a new meta model to a document. A JSON body must be passed:

{
  "metamodel": metamodel_id
}