Save default arrive-by time displayed in UI.

Fixes #518.
Do not require description for new scenario.

Fixes #522.  Field is allowed to be blank/null on django model.
Merge pull request #532 from flibbertigibbet/feature/save-default-arrive-by

Save default arrive-by time displayed in UI.
Merge pull request #533 from flibbertigibbet/feature/scenario-description-not-required

Do not require description for new scenario.