add EffectTag creation to API

[shapiromatron]

For animal.Endpoint, epi.Outcome, and epi.Result, the effect tag objects could not be created via API. This PR updates our API so that they can be added. Note that you cannot both create AND update a result tag with a single method.

To create a new tag, you can use a new API:

• POST /assessment/api/effect-tag/ with JSON like this: {name: foo, slug: foo}

Then, to add a tag to any item created, you use the name, for example, for animal.Endpoint:

• POST /ani/api/endpoint/ with {effects: ["foo"]}