Add a specific route for template_type=all

`all` is not a real template type, so for links to template folders that
apply to all template types we have a URL that looks like:
```
/services/<uuid:service_id>/templates/folders/<uuid:template_folder_id>
```

However Flask only generates this url when `url_for` is called with
`template_type=None`. If called with `template_type=all` then Flask will
generate a URL like
```
/services/<uuid:service_id>/templates/all/folders/<uuid:template_folder_id>
```

However attempting to load this URL will now 404, since `all` is not a
template type recognised by the regex introduced in
https://github.com/alphagov/notifications-admin/pull/3176

It would be nice to not have URLs with `all` in them at all, but since
people might have bookmarked them we need to support them indefinitely.

Also considered but decided against adding `all` to the set of template
types because it might cause other problems, for example attempting to
create a new template with a type of `all` would never work.

app/main/views/templates.py

@@ -108,6 +108,7 @@ def start_tour(service_id, template_id):
 @main.route("/services/<uuid:service_id>/templates", methods=['GET', 'POST'])
 @main.route("/services/<uuid:service_id>/templates/folders/<uuid:template_folder_id>", methods=['GET', 'POST'])
 @main.route("/services/<uuid:service_id>/templates/<template_type:template_type>", methods=['GET', 'POST'])
+@main.route("/services/<uuid:service_id>/templates/all/folders/<uuid:template_folder_id>", methods=['GET', 'POST'])
 @main.route("/services/<uuid:service_id>/templates/<template_type:template_type>/folders/<uuid:template_folder_id>", methods=['GET', 'POST'])
 @user_has_permissions()
 def choose_template(service_id, template_type='all', template_folder_id=None):

tests/app/main/test_permissions.py

@@ -422,7 +422,7 @@ def format_decorators(decorators, indent=8):
 def test_code_to_extract_decorators_works_with_known_examples():
     assert (
         'templates.choose_template',
-        ['main.route', 'main.route', 'main.route', 'main.route', 'user_has_permissions'],
+        ['main.route', 'main.route', 'main.route', 'main.route', 'main.route', 'user_has_permissions'],
     ) in list(
         get_routes_and_decorators(SERVICE_ID_ARGUMENT)
     )

tests/app/main/views/test_template_folders.py

@@ -268,6 +268,17 @@ def _folder(name, folder_id=None, parent=None, users_with_permission=None):
             [],
             'This folder is empty',
         ),
+        (
+            'folder_two – Templates – service one – GOV.UK Notify',
+            'Templates folder_two',
+            [{'template_type': 'all'}],
+            {'template_folder_id': FOLDER_TWO_ID, 'template_type': 'all'},
+            ['Email', 'Text message', 'Letter'],
+            [],
+            [],
+            [],
+            'This folder is empty',
+        ),
     ]
 )
 def test_should_show_templates_folder_page(