Misc changes, security patches, tests

Bugfixes:

• Fix index crashing when logged in.
• Fix all users being able to add/edit presentations/conferences by directly sending POST requests.

Changes:

• Conference is_virtual can now be changed after creation.
• Conference url can be specified during creation.
• Remove old conference_detail.html
• Index now shows most recent presentations and conferences and additionally also user presentations if logged in.
• Permission for adding/editing presentations/conferences now user.is_staff or user.is_superuser. user.coordinator is not functional due to not actually being implemented.
• All add/edit pages now consistently show/hide edit forms based on permissions.
• Redesign presentation detail page, add missing properties, more compact layout.

New tests for:

• Index
• Conference detail
• Conference add
• Conference edit
• Presentation detail
• Presentation add
• Presentation edit