Add `routablepageurl` template tag

It is similar to `pageurl`, but works with `RoutablePage`s. Functions
like a hybrid between `reverse` and `pageurl`. For example:

    {% load wagtailroutablepage_tags %}
    {% routablepageurl self "feed" %}
    {% routablepageurl self "archive" 2014 08 14 %}
    {% routablepageurl self "food" foo="bar" baz="quux" %}

docs/core_components/pages/advanced_topics/routable_page.rst

@@ -86,3 +86,19 @@ The ``RoutablePage`` class
         .. code-block:: python
 
             url = page.url + page.reverse_subpage('events_for_year', args=('2014', ))
+
+The ``routablepageurl`` template tag
+====================================
+
+.. currentmodule:: wagtail.contrib.wagtailroutablepage.templatetags.wagtailroutablepage_tags
+.. autofunction:: routablepageurl
+
+    Example:
+
+    .. code-block:: html+django
+
+        {% load wagtailroutablepage_tags %}
+
+        {% routablepageurl self "feed" %}
+        {% routablepageurl self "archive" 2014 08 14 %}
+        {% routablepageurl self "food" foo="bar" baz="quux" %}

wagtail/contrib/wagtailroutablepage/templatetags/wagtailroutablepage_tags.py

@@ -0,0 +1,24 @@
+from django import template
+
+
+register = template.Library()
+
+
+@register.simple_tag(takes_context=True)
+def routablepageurl(context, page, url_name, *args, **kwargs):
+    """
+    ``routablepageurl`` is similar to ``pageurl``, but works with
+    ``RoutablePage``\s. It behaves like a hybrid between the built-in
+    ``reverse``, and ``pageurl`` from Wagtail.
+
+    ``page`` is the RoutablePage that URLs will be generated from.
+
+    ``url_name`` is a URL name defined in ``page.subpage_urls``.
+
+    Positional arguments and keyword arguments should be passed as normal
+    positional arguments and keyword arguments.
+    """
+    request = context['request']
+    base_url = page.relative_url(request.site)
+    routed_url = page.reverse_subpage(url_name, args=args, kwargs=kwargs)
+    return base_url + routed_url

wagtail/contrib/wagtailroutablepage/tests.py

@@ -1,7 +1,8 @@
-from django.test import TestCase
+from django.test import TestCase, RequestFactory
 
-from wagtail.wagtailcore.models import Page
+from wagtail.wagtailcore.models import Page, Site
 from wagtail.tests.models import RoutablePageTest, routable_page_external_view
+from wagtail.contrib.wagtailroutablepage.templatetags.wagtailroutablepage_tags import routablepageurl
 
 
 class TestRoutablePage(TestCase):
@@ -80,3 +81,35 @@ class TestRoutablePage(TestCase):
         response = self.client.get(self.routable_page.url + 'external/joe-bloggs/')
 
         self.assertContains(response, "EXTERNAL VIEW: joe-bloggs")
+
+
+class TestRoutablePageTemplateTag(TestRoutablePage):
+    def setUp(self):
+        super(TestRoutablePageTemplateTag, self).setUp()
+        self.rf = RequestFactory()
+        self.request = self.rf.get(self.routable_page.url)
+        self.request.site = Site.find_for_request(self.request)
+        self.context = {'request': self.request}
+
+    def test_templatetag_reverse_main_view(self):
+        url = routablepageurl(self.context, self.routable_page,
+                              'main')
+        self.assertEqual(url, self.routable_page.url)
+
+    def test_templatetag_reverse_archive_by_year_view(self):
+        url = routablepageurl(self.context, self.routable_page,
+                              'archive_by_year', '2014')
+
+        self.assertEqual(url, self.routable_page.url + 'archive/year/2014/')
+
+    def test_templatetag_reverse_archive_by_author_view(self):
+        url = routablepageurl(self.context, self.routable_page,
+                              'archive_by_author', author_slug='joe-bloggs')
+
+        self.assertEqual(url, self.routable_page.url + 'archive/author/joe-bloggs/')
+
+    def test_templatetag_reverse_external_view(self):
+        url = routablepageurl(self.context, self.routable_page,
+                              'external_view', 'joe-bloggs')
+
+        self.assertEqual(url, self.routable_page.url + 'external/joe-bloggs/')