WIP: Support group based restriction

The data and the views are now "categorized". Each user, both staff and students, only sees the projects which are in one of their categories, e.g. "UROP Projects" or "DE Masters Projects". This also allows the staff to select the right category when creating or editing a project, they will see the dropdown selection below.

Staff with only one category don't need to do anything more than they used to do before, as only one category would appear and it would be selected by default. This system also implicitly allows to have a "default category" in case of staff members who have more than one category of projects.

app/forms/project.py

 from flask_wtf import FlaskForm
-from wtforms import BooleanField, StringField, TextAreaField
+from wtforms import BooleanField, SelectField, StringField, TextAreaField
 from wtforms.validators import DataRequired, ValidationError
 
 
@@ -8,9 +8,15 @@ class ProjectForm(FlaskForm):
     description = TextAreaField("Description", validators=[DataRequired()])
     is_student_proposal = BooleanField("Student Proposal", default=False)
     on_behalf = StringField("from", default=None)
+    category = SelectField("Category", coerce=str)
 
     def validate_on_behalf(self, field):
         if self.is_student_proposal.data and not field.data:
             raise ValidationError(
                 "Student proposer cannot be empty if project is marked as student proposal."
             )
+
+    def validate_category(self, field):
+        if field == self.category and field.data == "":
+            self.category.errors += "Please choose a category for the project."
+            raise ValidationError("Please choose a category for the project.")