LP1879517: Surveys shouldn't end before they begin

LP1879517: Surveys shouldn't end before they begin

To test:
1. Go to Admin > Local > Surveys.
2. Create a new survey.
3. Attempt to create a survey where the end date
comes before the start date.  Without this patch, you will get
no notice that this is invalid, and you can save the invalid
record.
4. Edit an existing survey.
5. Repeat step 3 while editing the existing survey.
6. Apply the patch.
7. Repeat steps 1-5.  Note that you now get a notice and
cannot save if the end date is before the start date.

This commit generalizes a validator already present in the booking
module, and corrects several small bugs related to the datetime-select
component.

Signed-off-by: Jane Sandberg <sandbergja@gmail.com>