REST service to add a new Rasa entity

[turnerrainer]

AS AN Architect I WANT TO have a REST service to add a new Rasa entity SO THAT there wouldn't be any duplicate services for it

Acceptance Criteria

• [x] Use POST endpoint /rasa/entities/add to add a new Rasa entity
• [ ] Use entities section of Rasa domain.yml as input
• [x] List output alphabetically

Examples

Sample input of domain.yml

entities:
  - asukoht
  - nimi
  - aadress

Output after making a request to add an entity called asutus

entities:
  - asukoht
  - nimi
  - aadress
  - asutus

[janyprus]

sample input for /rasa/entities/add

{
    "entity": "asukoht"
}

[turnerrainer]

@janyprus please link this issue with an appropriate pull request.

[janyprus]

Pull request #172

[turnerrainer]

Blocked due to #289

[Kristjan259]

Gives 300 error code when trying to add new entity