tasks.due_date and tasks.priority should not be strings

[stejskalleos]

In schema.rb, tasks.due_date and tasks.priority are both strings, but I think that due_date should be datetime and priority should be integer.

Models should have some validations:

• due_date should be validated that it can be only in the future
• priority should be limited to some selection, lets say from 1 to 3

UI should be updated accordingly to the changes

[nofaralfasi]

Created this PR solving this issue.