Fix: Make swagger doc compliant to OpenAPI (#3394)

This fixes issues in the swagger.json file. The motivation to do so is to be able to generate API clients using https://openapi-generator.tech Doing so will require further changes to the api spec, but this seems like a good first step since it is now "valid" according to editor.swagger.io and openapi-generator validate.

Fixes #2806

swagger/definitions/request/conversation/create_message.yml

@@ -1,9 +1,10 @@
 type: object
+required:
+  - content
 properties:
   content:
     type: string
     description: The content of the message
-    required: true
   message_type:
     type: string
     enum: ['outgoing', 'incoming']
@@ -18,4 +19,3 @@ properties:
   content_attributes:
     type: object
     description: attributes based on your content type
-