basically relays send us a lot of announce activities to share posts
but we don't care about those activities: it's db bloat and it increases
the shares count. keep a table keeping track of followed relays and
lazily skip activities by them, just fetch. IMPORTANT relays are only
loaded at startup, so if you subscribe to a new relay restart your
server once it accepts!!!
since this migration breaks all sqlite dbs, i changed the original
migration so that future ones won't panic when reaching here. note that,
if you are on sqlite, just `sqlite3 <your_db> .dump > backup.sql` and
then, after rebuilding and re-migrating db, `cat backup.sql | sqlite3 <your_db>`
