Add GraphQLTag model

[ruisaraiva19]

Resolves #70

This adds single and plural tags queries.

{
  tag(id: 1) {
    id
    name
  }
}

{
  tags {
    id
    name
  }
}

Using it on a model:

from django.db import models
from modelcluster.fields import ParentalKey
from modelcluster.contrib.taggit import ClusterTaggableManager
from taggit.models import TaggedItemBase
from wagtail.admin.edit_handlers import FieldPanel
from wagtail.core.models import Page
from grapple.models import GraphQLTag


class BlogPageTag(TaggedItemBase):
    content_object = ParentalKey(
        "BlogPage", related_name="tagged_items", on_delete=models.CASCADE
    )


class BlogPage(Page):
    # other fields
    tags = ClusterTaggableManager(through=BlogPageTag, blank=True)

    content_panels = Page.content_panels + [
        # other panels
        FieldPanel("tags"),
    ]

    graphql_fields = [
        # other graphql fields
        GraphQLTag("tags"),
    ]

[zerolab]

A couple of minor things, but excellent work!

[ruisaraiva19]

A couple of minor things, but excellent work!

@zerolab All updated!