Set a 'focus_date' for each Timeline

[jeremyboggs]

As of @4f7a884697a0a1a67e60678c57a1f40d76855f2c NeatlineTime starts at the earliest item on the timeline. However, we should decide on defaults and options. A few come to mind:

1. Date the timeline was created/modified
2. Date chosen by the user
3. Midpoint for all the events/items on the Timeline.

Seems like letting the user choose the focus date, with a few possible suggestions (like earliest date or midpoint) would be ideal. If the user doesn't set a date, it should default to the earliest date.

[jeremyboggs]

Status of this is dependent on the conversation on this ticket for Timeglider

[jeremyboggs]

Updates issue to remove references to Timeglider, and bring up-to-date with recent commits.

[csbailey5t]

Closed in f81ed8f