added deletion functionality for concept mappings

this is optional (via `editable`) because we don't want it in view mode

note that this required
* abandoning the convenience wrapper to avoid duplicating the signature
* adding classes for more semantic markup

also, ideally we'd move the concept mappings data structure into
dedicated class with a proper API (e.g. `removeItem(uri, matchType)`)
added textarea synchronization to concept mapping manager

fairly primitive, but pragmatic
created label resolver

resolves link labels based on their URL

still incomplete (cf. debugging remnants)