Europe PMC Publication annotations

[SandyRogers]

This PR:

• adds a detail action endpoint on Publications, to fetch and format Europe PMC paper annotations for the Publication.
• does not add a Model (no annotations data is stored in MGnify db).
  • this means we should not expect to use/include annotations on a Publication list endpoint
  • this means if Europe PMC is down, our detail endpoint will 404 (but the main Publications list/detail endpoints will be fine)

TODO in future:

Action endpoints do not appear in the Browsable API / API schema currently. It would be nice if it were possible to add a model-less RelationshipViewSet to Publications, so that you'd get some JSON like:

"relationships": {
    "studies": {
        "links": {
            "related": "http://localhost:8000/v1/publications/29312205/studies"
        }
    },
    "samples": {
        "links": {
            "related": "http://localhost:8000/v1/publications/29312205/samples"
        }
    },
+  "annotations": {
+      "links": {
+          "related": "http://localhost:8000/v1/publications/29312205/annotations"
+      }
+  }
},

This isn't possible (I think it is this issue).

Another currently-impossible but nice way would be to use something like .../publications/<pubmedid>?include=annotations, with annotations as a calculated field. This is probably the thing to do, but first we need to change our DRFJA implementation.

[mberacochea]

It looks good :). Only those comments/questions. I think we should deploy this in dev and give it a try