Added child and sibling Articles selection tests.

Decided to put those in contentmodeltests.py instead of contentaccesstests.py
because I've created new methods in the Article model class for getting
lists of an Article's children and sibling items.
Bugfix in ContentModels.py

Content.save() still forces naming conventions even when editing items.
Cleanups in views/Public.py and the tests; also in models/ContentModels.py

New behaviours:
1. When a Section has no homepage (i.e., there are no Articles posted
there because any Articles were moved or deleted), one is automatically
created when the Section is accessed publicly. We might want to do this
every time the Section is selected but we'll get there.

2. A Section that has Articles posted under it can now be deleted.
All Articles posted in a Section that will be deleted will also be
deleted.

3. Related to (maybe conflicting with) #1, Section homepages are no
longer allowed to be moved. However, they can be deleted. We might
want to rethink this.