improved site navigation
- can get to all project visit types, sites, panels, and subjects from the project view
- can also add any of the above from the project view

view_project_subjects.html:
- new template for viewing project subjects

models.py:
- new Project methods 'get_visit_type_count' and 'get_panel_count'

urls.py:
- organized routes by parent model type
- added route for viewing project subjects

views.py:
- new view for viewing subjects
- modified the templates for the project views

base.html:
- removed link to all projects since this is exactly where the home link goes