Implement complete labels API (create/delete/update)

##### What does this MR do?

As labels got their color feature, the API also needs some :heart:. 

This PR adds the following features:

* Create a new label via API
* Delete existing labels via API
* Update existing label via API

##### Are there points in the code the reviewer needs to double check?

Creating a new label is refactored out to a service.

##### Why was this MR needed?

Currently, the API can only **get** labels

@jvanbaarsen Could you review :innocent:
What about adding a scope to the uniqueness? like: `uniqueness: { scope: :project_id }` (Not sure if valid syntax)