Add WebID support

Really nice feature would be to add a clean WebID support for wordPress users (creation of certificates and handling of cert URLs)

Discussion ongoing with Angelo, creator of: https://github.com/angelo-v/wp-linked-data on the strategy to achieve this (here: https://github.com/angelo-v/wp-linked-data/issues/2 )

Start by reading the issue and the linked discussion in angelo-v/wp-linked-data#2 to understand the proposed strategy. The scope includes WebID support for WordPress users, certificate creation, and handling certificate URLs; the work is complete only when those requirements and an agreed implementation strategy are defined and supported.

Written by the indexing model from the issue text.