Add GET routes for /user/tags and /user/tags/{id}

An operation that is useful for tools to be able to do is to list a
user's tags without having to pull down the entire user object (which is
likely significantly larger than the tags). This commit adds a route to
GET /user/tags, to get a list of the user's tags, as well as a GET
/user/tags/{id} to get the details of a particular tag.

This commit also includes tests for the two new routes.

test/api/user/GET-user_tags.test.js

@@ -0,0 +1,20 @@
+import {
+  generateUser,
+  requester,
+} from '../../helpers/api.helper';
+
+describe('GET /user/tags', () => {
+  let api, user;
+
+  beforeEach(() => {
+    return generateUser().then((usr) => {
+      user = usr;
+      api = requester(usr);
+    });
+  });
+
+  it('gets the user\'s tags', () => {
+    return expect(api.get('/user/tags'))
+      .to.eventually.eql(user.tags);
+  });
+});

test/api/user/GET-user_tags_id.test.js

@@ -0,0 +1,25 @@
+import {
+  generateUser,
+  requester,
+} from '../../helpers/api.helper';
+
+describe('GET /user/tags/id', () => {
+  let api, user;
+
+  beforeEach(() => {
+    return generateUser().then((usr) => {
+      user = usr;
+      api = requester(usr);
+    });
+  });
+
+  it('gets a user\'s tag by id', () => {
+    return expect(api.get('/user/tags/' + user.tags[0].id))
+      .to.eventually.eql(user.tags[0]);
+  });
+
+  it('fails for non-existent tags', () => {
+    return expect(api.get('/user/tags/not-an-id'))
+      .to.eventually.be.rejected.with.property('code', 404);
+  });
+});