Use enums (e.g. class roles, group status, group action type)

[knrafto]

We have a lot of stringly-typed enums, and this can be error-prone. We should use enums instead.

Using enums could have other benefits, such as a method that makes a human-readable string of the enum (e.g. Role.lab_assistant -> "lab assistant").

Here's how to hook it up to SQLAlchemy (stolen from the OH app):

class EnumType(db.TypeDecorator):
    impl = db.String(255)

    def __init__(self, enum_class):
        super(EnumType, self).__init__(self)
        self.enum_class = enum_class

    def process_bind_param(self, enum_value, dialect):
        return enum_value.name

    def process_result_value(self, name, dialect):
        return self.enum_class[name]

    @property
    def python_type(self):
        return self.enum_class

...

Role = enum.Enum('Role', ['student', 'lab assistant', ...])

class MyModel(db.Model):
    my_enum_column = db.Column(EnumType(Role), nullable=False, index=True)

[Sumukh]

This also looks easier if we need to update the table/run a migration.