e2e: Wait for "Create" link in beforeEach

This should resolve the following failures:

- https://travis-ci.org/mbland/custom-links/jobs/260701744
- https://travis-ci.org/mbland/custom-links/jobs/260701972

This may also finally fix the flakiness observed in earlier builds, as
the problem was likely that the first `activeElement()` call came before
the expected element became active, considering that the view is set
by `loadApp()` some time after the DOMContentLoaded event. The hint was
the common failure message from both of the above builds:

```
 1) End-to-end test shows the no-links message before any links created:

      AssertionError: expected '| My links | New link | Log out\nCustom
Links' to deeply equal 'Create a new custom link'

      + expected - actual

      -| My links | New link | Log out
      -Custom Links
      +Create a new custom link
```

tests/end-to-end/end-to-end.js

@@ -43,7 +43,8 @@ test.describe('End-to-end test', function() {
   })
 
   test.beforeEach(function() {
-    return driver.get(url)
+    driver.get(url)
+    return waitForActiveLink('Create a new custom link')
   })
 
   test.afterEach(function() {
@@ -91,7 +92,6 @@ test.describe('End-to-end test', function() {
   }
 
   test.it('shows the no-links message before any links created', function() {
-    activeElement().getText().should.become('Create a new custom link')
     activeElement().getAttribute('href').should.become(url + '#create')
     activeElement().click()
     driver.wait(until.urlIs(url + '#create'))
@@ -108,11 +108,14 @@ test.describe('End-to-end test', function() {
   })
 
   test.it('creates a new short link', function() {
+    // Rather than click on the active "Create a new custom link" link, let's
+    // make sure we can navigate to "New link" in the nav bar as expected.
     activeElement().sendKeys(Key.chord(Key.SHIFT, Key.TAB))
     activeElement().sendKeys(Key.chord(Key.SHIFT, Key.TAB))
     activeElement().getText().should.become('New link')
     activeElement().click()
     driver.wait(until.urlIs(url + '#create'))
+
     activeElement().sendKeys('foo' + Key.TAB)
     activeElement().sendKeys(targetLocation + Key.TAB)
     activeElement().sendKeys(Key.ENTER)