Fixed various bugs
Some important concepts to bear in mind:
* Notification: Associated with activities, won't be reconstructed
together with objects, can be thought of as transient
* Attention: Associated with objects, will be reconstructed with them, can
be thought as persistent
* Notifications and Attentions have no direct implications.
* Mentions are a specific form of attentions in notes, leads to the creation of Attentions.
Finally,
Potential PHP issue detected and reported: https://github.com/php/php-src/issues/8199
`static::method()` from a non static context (such as a class method) calls `__call`, rather than
the expected `__callStatic`. Can be fixed by using `(static fn() => static::method())()`, but the
usage of the magic method is strictly unnecessary in this case.
Replaces Note content with a tombstone, removes any attachment relations and decrements their lives (possibly even removing the attachment), and creates the respective activity
