User facing changes:

- Iteration title won't be included in notification emails.
- Board list title for iteration will display the iteration's period.
- Iterations created in advance for a cadence won't have titles (behind
  FF.)

Internal changes:

- Title is no longer a required attribute for Iteration model

Timebox concern previously hosted the validation logic for title
attribute for both Milestone and Iteration.
Since title is only required and needs to be validated for milestone,
the validation logic is moved from Timebox concern into Milestone model.
For Iteration model, only sanitization needs to be done for title.

- User must specify a title unless using the FF iteration_cadences when
creating/updating a new iteration.

Changelog: changed

Apply backend reviewer suggestion