Add a /entities endpoint to the API

[MattiSG]

Returns a JSON object listing all modelled entities where the property names is the key property of the entity and where the value is an Object with the following properties:

• String plural. Mandatory.
• String documentation, coming from the entity docstring. Defaults to "".
• Object roles if the entity is not a person (in case the entity is a person, roles must be falsy), where the name is the key property and where the value is an object with the following properties:
  • String plural. Mandatory.
  • Boolean mandatory. Defaults to false.
  • Number max (if one is defined).

subroles are not to be exposed yet.

Related to openfisca/legislation-explorer#80.

[guillett]

This endpoint is required to create a proper UI.

It would be a great addition to the web API.

[fpagnoux]

For more discussion about subroles: #560 (comment)