Add model method used in creating static selectboxes

Signed-off-by: Prashant Shah <prashah360@gmail.com>
Add priority and difficulty levels for goals

- Add validation for the respective database fields
- Add model methods to return all the possibles values for
these fields

Signed-off-by: Prashant Shah <prashah360@gmail.com>
Add view fields for difficulty and priority

Signed-off-by: Prashant Shah <prashah360@gmail.com>