* add external_level and internal_level
* remove unnecessary debug derive on LinkDef
* clarify doc comment about link check levels
* simplify link checker logging
* add missing warn prefix
* simplify link level logging, remove "Level" from linklevel variants
* remove link level config from test site
* switch back to using bail! from get_link_domain
* move console's deps to libs
* remove unnecessary reference
* calling console::error/warn directly
* emit one error, or one warning, per link checker run
* various link checker level changes
* add docs about link checker levels
* remove accidentally committed test site
* remove completed TODO
