This should fix nulls on explorer lookups inputed by postman after generate_followers/getSubscribers, that I think were caused by calling common_profile_uri that, curiously, only handles local profiles
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