Put ‘add new’ button at top of page

One of the most frequent tasks done on this page is adding a new
branding.

Current to do this you have to:
- scroll to the bottom
- scan for the ‘Create a new email branding’ option that visually looks
  just like all the other brandings
- submit a form

This commit makes change it to one clearly differentiated button at the
top of the page. This is consistent for how we let users add templates
and team members.

tests/app/main/views/test_email_branding.py

@@ -25,14 +25,15 @@ def test_email_branding_page_shows_full_branding_list(
 
     assert normalize_spaces(
         page.select_one('h1').text
-    ) == "Select an email branding to update or create a new email branding"
+    ) == "Email branding"
+
+    assert page.select_one('.column-three-quarters a')['href'] == url_for('main.create_email_branding')
 
     first_label = radio_labels[0]
     assert normalize_spaces(first_label.text) == 'org 1'
     assert brand_names == [
-        'org 1', 'org 2', 'org 3', 'org 4', 'org 5', 'Create a new email branding']
-
-    assert normalize_spaces((radio_labels[-1]).text) == 'Create a new email branding'
+        'org 1', 'org 2', 'org 3', 'org 4', 'org 5'
+    ]
 
 
 def test_edit_email_branding_shows_the_correct_branding_info(