Add additional study document types. (#421)

This commit adds two new types of study documents: `oral exams` and `study guides`.

- `oral exams` are introduces mainly for maintenance. We received complaints that it is hard to get an overview of available oral exams because it is currently impossible to differentiate them from normal exams.
- `study guides` are mainly intended for the AMIV PVK, but also for similar amiv-hosted courses that provide guides in form of scripts or presentations to students. They are different from `cheat sheets`, as they are much more extensive. And they are different from `lecture documents`, as they are neither official documents, nor necessarily follow the lecture structure (e.g. a PVK might cover the topics of a lecture, but present them in different order and context). Finally, there is a need for students to find study guides (compared to generic lecture notes) for their exam preparations.

Closes #420.

amivapi/studydocs/model.py

@@ -184,8 +184,9 @@ def _validate_allow_summary(self, *args, **kwargs):
                 'type': 'string',
                 'nullable': True,
                 'default': None,
-                'allowed': ['exams', 'cheat sheets', 'lecture documents',
-                            'exercises'],
+                'allowed': ['exams', 'oral exams',
+                            'cheat sheets', 'study guides',
+                            'lecture documents', 'exercises'],
                 'allow_summary': True,
             },
             'course_year': {