feature/metafeeds: Support for canteen meta feeds and index feed (#2)

* feature/metafeeds: Restructured API, splitted menu and meta feed

* feature/metafeeds: add field 'key' to Canteen

* feature/metafeeds: added helper function for reverse urls

* feature/metafeeds: added menu feed url to meta feed

* feature/metafeeds: added missing test

(added: testing meta feed rendering in retrievaltest)

* feature/metafeeds: added feed index page

* whitespace fix

* feature/metafeeds: added alias urls

* feature/metafeeds: moved meta feed default url

* feature/metafeeds: moved index generation to views

* feature/metafeeds: inlined builder instance creation

* feature/metafeeds: removed unused 'menu' parameter

* feature/metafeeds: replaced 'reverse' module with Flask's url_for

* feature/metafeeds: removed unused import

* feature/metafeeds: removed unused function parameters

* feature/metafeeds: moved menu feed url generation to views.py

* feature/metafeeds: cleanup debug prints

removing debug prints that should never have been committed.

tests/resources/meta_output.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<openmensa version="2.1" xmlns="http://openmensa.org/open-mensa-v2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://openmensa.org/open-mensa-v2 http://openmensa.org/open-mensa-v2.xsd">
+  <canteen>
+    <name>Mensa Griebnitzsee</name>
+    <address>August-Bebel-Straße 69, Haus 6</address>
+    <city>14482 Potsdam</city>
+    <feed name="full" priority="0">
+      <schedule dayOfMonth="*" dayOfWeek="*" hour="8-18" minute="0" retry="30 1"/>
+      <url>canteens/griebnitzsee/menu</url>
+    </feed>
+  </canteen>
+</openmensa>