Refs #314 -- Read-only REST API (#513)

* initial commit

* API auth

* Hierarchical URLs

* Add session auth

* Strong hierarchy

* Add filters

* Add i18n fields, questions

* More viewsets and serializers

* Ticket download

* Add OrderPosition serializer

* View-level permissions

* More tests

* More tests

* Add basic API docs

* Add REST API to docs frontpage

* Tests for order endpoints

* Add invoice tests

* Voucher and waitinglist tests

* Doc draft

* order docs

* Docs on all viewsets

* Disable DRF docs, style sphinx, style browsable API

* Fix tests

* deprecated imports

* Test foo

* Attendee names

* Fix migration problems

* Remove browsable API, plugin integration

* Doc fixes

doc/_templates/index.html

@@ -38,6 +38,22 @@
         </div>
     </div>
     <div class="clearfix"></div>
+    <div class="sectionbox">
+        <div class="icon">
+            <a href="api/index.html">
+                <span class="fa fa-plug fa-fw"></span>
+            </a>
+        </div>
+        <div class="text">
+            <a href="api/index.html">
+                <strong>REST API</strong>
+            </a>
+            <p>
+                Documentation and reference of the RESTful API exposed by pretix for interaction with external
+                components.
+            </p>
+        </div>
+    </div>
     <div class="sectionbox">
         <div class="icon">
             <a href="development/index.html">
@@ -52,6 +68,7 @@
                 pretix.</p>
         </div>
     </div>
+    <div class="clearfix"></div>
     <div class="sectionbox">
         <div class="icon">
             <a href="plugins/index.html">
@@ -65,7 +82,6 @@
             <p>Documentation and details on plugins that ship with pretix or are officially supported.</p>
         </div>
     </div>
-    <div class="clearfix"></div>
     <div class="sectionbox">
         <div class="icon">
             <a href="contents.html">