Add `isSidebar` utility

The client application has several existing tests to see if the app is
“in sidebar mode” currently. This utility is a first step to harmonizing
this and making things consistent.

src/sidebar/util/is-sidebar.js

+'use strict';
+
+/**
+ * Is the instance of the application in the current `window_` within a
+ * sidebar (vs. single-annotation or stream view)?
+ */
+const isSidebar = (window_ = window) => {
+  return !(
+    window_.location.pathname.startsWith('/stream') ||
+    window_.location.pathname.startsWith('/a/')
+  );
+};
+
+module.exports = isSidebar;

src/sidebar/util/test/is-sidebar-test.js

+'use strict';
+
+const isSidebar = require('../is-sidebar');
+
+describe('sidebar.utils.is-sidebar', () => {
+  [
+    '/a/random-annotation-id',
+    '/stream/',
+    '/stream',
+    '/a/',
+    '/a/foo.html',
+  ].forEach(fakePath => {
+    it(`returns false if window.location is in stream or single-annotation view (${fakePath})`, () => {
+      const window_ = {};
+      window_.location = new URL(`http://www.example.com${fakePath}`);
+      assert.isFalse(isSidebar(window_));
+    });
+  });
+
+  ['/whatever', '/anything-else', '/a', '/apples/', '/app.html'].forEach(
+    fakePath => {
+      it(`returns true if window.location is not in stream or single-annotation view (${fakePath})`, () => {
+        const window_ = {};
+        window_.location = new URL(`http://www.example.com${fakePath}`);
+        assert.isTrue(isSidebar(window_));
+      });
+    }
+  );
+});