Add field to Events model to relate to SourceItems

- Add sources field to Event
- Add sources api field EventResource
- Add events api field SourceItemResource
Creating an event from a source relates the two
List assigned events in sourceitem edit view

- Add assigned events to sourceitem edit template
- Create helper to sum durations
Hover sourceitem makes assigned events react

- Fire events when source items hovered
- Catch highlight events and update event model
- Add Styles for event-highlight
Show assigned source-items
Issue #39: Show Events and SourceItem relationship

As a user it's hard to see which sourceitems have been assigned to
events

- Add field to Events model to relate to SourceItems
- Creating an event from a source relates the two
- Hover sourceitem makes assigned events react to show which ones they are
- List assigned events in sourceitem edit view
- Show assigned sourceitems in list