Adjust APIDocsView static views to use base_endpoint

[markchoward7]

#571

Adjusts the calls to url_for within the APIDocsView to utilize the base_endpoint variable to account for setting that configuration.

[markchoward7]

@billyrrr Can this get looked at? I've been using an ugly monkeypatch to use this in my environment for some time now.

To be clear on the issue, I'm trying to host flasgger at multiple endpoints (/api/v1 and /api/v2) and I use the following config options when declaring my Swagger instances (version number adjusted for each of course).

{
  "headers": [],
  "specs_route": "/api/v1",
  "endpoint": "flasgger-v1",
  "specs": [
      "endpoint": "apispec_v1",
      "route": "/apispec_v1.json",
      "rule_filter": lambda rule: version in rule.endpoint,
      "model_filter": lambda tag: True,
}

Without this change, I get an error trying to access flasgger werkzeug.routing.exceptions.BuildError: Could not build url for endpoint 'flasgger.static' with values ['filename']. Did you mean 'flasgger-v1.static' instead?

I think that APIDocsView.get should be utilizing the endpoint configuration option to allow this to work.

[billyrrr]

LGTM