Explore representing classic-style bonds as `ActiveEffect`s

[rsek]

As of v11, ActiveEffects have a description field; at present, the bond object is {name: string, notes: string}, so the data migration would be pretty simple.

Potential benefits

• bonds no longer tied to a specific item -- instead, 1 embedded document = 1 bond
• take advantage of FVTT's various document methods and other utilities
• use ActiveEffect#icon to provide a portrait or other image of the bond
• represent system.bond ticks with an EffectChange; it'd be easy to adjust the number of ticks added by a specific bond
• leverage code for shared ActiveEffects (in #770 ) to propagate shared bonds

Component changes

It's a bit of a departure from the current bonds manager, but I think the simplest solution would parallel progress item management: the list/overview of bonds gets its own tab, and editing an individual bond opens a new window.

The MVP could borrow heavily from our existing item list components. For the list view, the individual bond components should probably show only a name and an icon, arranged in a multiple-column thumbnail layout.