improve lexical relations

[dwhieb]

Divide lexical relations into:

1. cross-references (lexeme level)
2. lexical relations (sense level)

This mirrors the organization in FLEx.

Update the schemas to use the following properties

• label: the label to display before the cross-reference
• plural: the label to display before the cross-reference, if there are more than one
• relation the name of the cross-reference relation (e.g. Synonym, Part/Whole, Adjective Scale)
• relationType: unidirectional|bidirectional|open set|enumerated set
• members: either
  a. [enumerated sets] an object hash whose keys are the member slots (e.g. positive, comparative, and superlative)
  b. [sets] an array of IDs of items in that set (may include the ID of the current item)