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:
We create services to prevent callbacks mess and keep controllers clean.
We dont need services for Labels because all operation match basic AR (`create`, `update_attributes`).  
In this particular case instead of one-line `project.labels.create(params)` we get a huge amount of unnecessary code. 

Create label: 

```
project.labels.create(params)
```

Update label

```
label = project.labels.find_by(name: 'feature')
label.update_attibutes(params) ... 
```