This adds a requirement for all definitions that have foreign keys to also
require indices for all source (local) attributes mentioned in foreign keys.
MariaDB/MySQL creates indices for source attributes automatically, so this
serves as a way to get rid of those automatic indices and create clean explicit
ones instead.
In PostgreSQL, most of the time, indices on the source are necessary to
decrease performance penalty of foreign keys (like in MariaDB), but they aren't
created automatically, so this serves to remove that difference between
PostgreSQL and MariaDB.
This is not the same as the one in https://notabug.org/diogo/gnu-social-activitypub-plugin
Differences to the first "release"
-> Doesn't use guzzle nor has any composer dependencies
-> Supports HTTP Signatures
-> Has basic l10n/i18n
-> Some minor bug fixes
