Switch Project.tags to django-taggit

[taein6765]

This commit closes #64.

[sumanthratna]

TLDR: let's come back to this later

---

to merge this, we need to remove the tag field from all fixtures, which I don't want to do yet.

TaggableManager isn't deserialized, I think because of this line: https://github.com/jazzband/django-taggit/blob/9d9ca4b36a09ec7b4cb0c823be9f90ff2089701e/taggit/managers.py#L441

proof that `tags` isn't deserialized in fixtures

output of:

poetry run python manage.py shell --command "from judge.models import Project; mod = Project.objects.create(name='apple', location='tree', description='desc', tags=['fruit', 'red']); from django.core import serializers; print(serializers.serialize('json', (mod,)))"

is:

[{"model": "judge.project", "pk": 1, "fields": {"name": "apple", "location": "tree", "description": "desc", "link": "", "overall_mean": 0.0, "overall_variance": 1.0, "innovation_mean": 0.0, "innovation_variance": 1.0, "functionality_mean": 0.0, "functionality_variance": 1.0, "design_mean": 0.0, "design_variance": 1.0, "complexity_mean": 0.0, "complexity_variance": 1.0, "numberOfVotes": 0, "timesSeen": 0, "timesSkipped": 0, "prioritize": false, "active": true}}]

We can still keep tag data in fixtures, but will take more work:

1. poetry run python manage.py loaddata 'fixture.json' --ignorenonexistent
2. poetry run python manage.py assigntags 'fixture.json' # we need to create this script