Consistent endpoint structure and naming

[ts5746]

Problem

Currently, endpoint naming is not fully consistent, and contain actions:

• /originators/tokens combines two different resource types
• /messages/decode, /messages/validate are named as actions on collections, but actually operate on a single message not being a resource, just like/token/create and signature/verify

Rationale

The API naming and structure should be consistent.

Solution

Change the endpoint structure and naming as follows:

• use plurals only for collections of resources
• create a new /tokens resource type endpoint, replacing /originators/tokens
• change endpoints names to singular if singletons, i.e. not containing or operating on any resources: /message, /token, /signature
• actions may only be defined on a singleton or a single resource, e.g. /message/send, /token/create, /signature/verify, /blockchains/{blockchain}/scan

[ts5746]

Also check for consistent use of the resource property of the meta object in the response body.