Fixed Issue #83

An error message now appears when the user has entered more than 255
characters. The submit button will also stay disabled until the error
is resolved.
Partially fixed Issue #82

- when an invalid time period is submitted, the Save button stays disabled.
The reason is the $invalid property for the form becomes true. Fixed it
by using ng-if instead of ng-show
- changed time validation - answer_start cannot be equal to answer_end