Need to initialize AttributeFields with link destinations specified

[drusepth]

Right now we're creating the initial AttributeFields for an account based on the original <content>.yml files, which don't include the types of pages to link to. Instead, we're inferring those page types from the old-style links/relations.

Before we can remove those relations (and associated tables, etc), we'll need to add the those link destinations to all of the default field specs -- and we should probably just use those to create new-style links upon initialization.