add_visit_type.html, view_project_visit_types.html:
- new templates for adding and viewing project visit types

forms.py:
- new ModelForm for ProjectVisitType Model
- SampleForm: override __init__ to restrict visit types to ones associated with the project

models.py:
- new model for ProjectVisitType, FK to Project
- added FK for ProjectVisitType to Sample model, is optional (null=True, blank=True)

urls.py:
- new routes for the 2 project visit type templates

views.py:
- new view for adding and viewing project visit types

base.html:
- updated with link for viewing all project visit types