Add support for day spanning

Steps to reproduce with Smart Date

Create smart date with recurring rule
Save entity
Edit entity
Click on manage instances
Click override on one instance
Change the end date to span more than one day

Multi-day spanning event are implemented in this commit.

I use a CSS-only solution which adds classes on each instance:

• .is-multi
• .is-multi--first
• .is-multi--middle
• .is-multi--last

Tested with date range fields and smart date recurring field with overrides.

Code currently only on 2.0.x branch.

Front end improvements have taken place in MR!11.

A new JS library takes care of resizing and positioning multi day events properly in the calendar table.

See attached screenshot.

Automatically closed - issue fixed for 2 weeks with no activity.