Add options for setting interval size for each band

[jeremyboggs]

The intervals for timeline bands should be options the user can set for each timeline. This would require adding a database column and saving these options to the NeatlineTimeTimeline record. It would also require updating NeatlineTime::loadTimeline in neatline-time-scripts.js to accept those options as arguments.

[erochest]

This would involve:

• schema change;
• migration;
• admin config form;
• passing the setting to the browser; and
• changes in the timeline component.