WIP tags: REST API addition

Addition of rest api for tags.
Still work in progress.
`m` doesn't mean anything, drop it.
```python
for err in super(TagsRestValidator, self).get_errors():
    yield err
base_error = dict(code=..., message=...)
for key, error in self.errors.items():
    if error == "out of bounds":
        yield dict(base_error, field=key)
```