Fix followers synchronization mechanism not being triggered on mentions (#15026)
Fix followers synchronization mechanism not being triggered on mentions (#15026)

e.g. if someone on an instance that previously had followers gets mentioned
in a private toot, before this PR, they would not receive a
Collection-Synchronization header and may show the toot to the former followers
in addition to the mentioned person.
tootcafe/mastodon,yukimochi/mastodon,masto-donte-com-br/mastodon,abcang/mastodon,cobodo/mastodon,primenumber/mastodon,Nyoho/mastodon,musashino205/mastodon,imas/mastodon,musashino205/mastodon,ykzts/mastodon,ikuradon/mastodon,h3zjp/mastodon,MitarashiDango/mastodon,lindwurm/mastodon,h3zjp/mastodon,danhunsaker/mastodon,primenumber/mastodon,h3zjp/mastodon,imas/mastodon,MitarashiDango/mastodon,musashino205/mastodon,maa123/mastodon,yukimochi/mastodon,lindwurm/mastodon,maa123/mastodon,imas/mastodon,Ryanaka/mastodon,cobodo/mastodon,im-in-space/mastodon,Nyoho/mastodon,Ryanaka/mastodon,pinfort/mastodon,theoria24/mastodon,glitch-soc/mastodon,danhunsaker/mastodon,im-in-space/mastodon,tootcafe/mastodon,cobodo/mastodon,tootsuite/mastodon,Ryanaka/mastodon,lynlynlynx/mastodon,gol-cha/mastodon,MitarashiDango/mastodon,masto-donte-com-br/mastodon,Nyoho/mastodon,maa123/mastodon,Kirishima21/mastodon,yukimochi/mastodon,koba-lab/mastodon,Nyoho/mastodon,ykzts/mastodon,lynlynlynx/mastodon,abcang/mastodon,tootsuite/mastodon,tootsuite/mastodon,theoria24/mastodon,ykzts/mastodon,yi0713/mastodon,Kirishima21/mastodon,pinfort/mastodon,tootcafe/mastodon,im-in-space/mastodon,tri-star/mastodon,h3zjp/mastodon,masto-donte-com-br/mastodon,tri-star/mastodon,pinfort/mastodon,tri-star/mastodon,glitch-soc/mastodon,lynlynlynx/mastodon,koba-lab/mastodon,maa123/mastodon,koba-lab/mastodon,yukimochi/mastodon,gol-cha/mastodon,danhunsaker/mastodon,yi0713/mastodon,yi0713/mastodon,theoria24/mastodon,cobodo/mastodon,tootsuite/mastodon,masto-donte-com-br/mastodon,Kirishima21/mastodon,glitch-soc/mastodon,glitch-soc/mastodon,tootcafe/mastodon,ikuradon/mastodon,danhunsaker/mastodon,Ryanaka/mastodon,ikuradon/mastodon,lynlynlynx/mastodon,primenumber/mastodon,Kirishima21/mastodon,gol-cha/mastodon,pinfort/mastodon,MitarashiDango/mastodon,gol-cha/mastodon,koba-lab/mastodon,yi0713/mastodon,ykzts/mastodon,lindwurm/mastodon,ikuradon/mastodon,abcang/mastodon,musashino205/mastodon,lindwurm/mastodon,im-in-space/mastodon,imas/mastodon,abcang/mastodon,tri-star/mastodon,primenumber/mastodon,theoria24/mastodon