Add VulnerabilityEntity w/ JSON schema tests

ee/app/serializers/vulnerability_entity.rb

+# frozen_string_literal: true
+
+class VulnerabilityEntity < Grape::Entity
+  expose :id
+  expose :title
+  expose :description
+
+  expose :state
+  expose :severity
+  expose :confidence
+
+  expose :project, using: ::ProjectEntity
+
+  expose :author_id
+  expose :updated_by_id
+  expose :last_edited_by_id
+  expose :closed_by_id
+
+  expose :start_date
+  expose :due_date
+
+  expose :created_at
+  expose :updated_at
+  expose :last_edited_at
+  expose :closed_at
+end

ee/spec/fixtures/api/schemas/vulnerability.json

+{
+  "type": "object",
+  "required": ["title", "state", "confidence", "severity", "project", "author_id"],
+  "properties": {
+    "title": {
+      "type": "string"
+    },
+    "description": { "type": ["string", "null"] },
+    "state": { "type": "string", "enum": ["opened", "closed"] },
+    "severity": {
+      "type": "string",
+      "enum": ["undefined", "info", "unknown", "low", "medium", "high", "critical"]
+    },
+    "confidence": {
+      "type": "string",
+      "enum": [
+        "undefined",
+        "ignore",
+        "unknown",
+        "experimental",
+        "low",
+        "medium",
+        "high",
+        "confirmed"
+      ]
+    },
+    "project": {
+      "required": ["id", "name", "full_path", "full_name"],
+      "id": {
+        "type": "integer"
+      },
+      "name": {
+        "type": "string"
+      },
+      "full_path": {
+        "type": "string"
+      },
+      "full_name": {
+        "type": "string"
+      }
+    },
+    "author_id": { "type": "integer" },
+    "updated_by_id": { "type": ["integer", "null"] },
+    "last_edited_by_id": { "type": ["integer", "null"] },
+    "closed_by_id": { "type": ["integer", "null"] },
+    "start_date": { "type": ["date", "null"] },
+    "due_date": { "type": ["date", "null"] },
+    "created_at": { "type": "date" },
+    "updated_at": { "type": "date" },
+    "last_edited_at": { "type": "date" },
+    "closed_at": { "type": "date" }
+  }
+}

ee/spec/serializers/vulnerability_entity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe VulnerabilityEntity do
+  let(:vulnerability) do
+    create(:vulnerability)
+  end
+
+  let(:entity) do
+    described_class.represent(vulnerability)
+  end
+
+  subject { entity.to_json }
+
+  it { is_expected.to match_schema('vulnerability', dir: 'ee') }
+end