Add support for "tagging" / "pinging" users

[tkuhn]

So, users can get notified about nanopublications that affect them, e.g. possible issues.

[Daniel-Mietchen]

A nanopub asking for this feature: https://w3id.org/np/RACGRpLtkjdvNAQFau4vHHw8-9iCiXLc_eKX6IqT3wc74 .

Example nanopubs of a kind in which such a feature could have been used:

• https://w3id.org/np/RALDUNQ8Sdua9MiklhKJXrIS5m9qgMzEIC0UlAE_5Ge7U
• https://w3id.org/np/RAtnOTzUXLYCVzEj5q0W3CMhdzDuPWE5fHnhEtjTPpS1k
• https://w3id.org/np/RAFSlOn0-9gepmHlzNjBTFkkfvNFWI1sKw7_7UEfMuvhE
• https://w3id.org/np/RAxtftx5MOwgLHwXUFxgwtuNFPcFFmDR1lXePG99VYWmo