Reactive pos-label and pos-description

[jg10-mastodon-social]

Acceptance criteria:

• Thing is refactored to use PodOS Store instead of rdflib.js store
• Observable for anyValue, observing subject+predicates
• Observable for description: wrapper around anyValue
• Observable for label: wrapper around anyValue
• Pos-label uses new observable
• Pos-description uses new observable

I'm suggesting grouping these given that supporting both label+description seems to have substantial overlap and their effect is primarily seen through the web components.