Adds ActivityPub support

This MR implements federation support via the ActivityPub protocol.

Support for the following features:

• WebFinger user discovery;
• Support asymmetric relations (follow/unfollow) across networks;
• Support scraps and replies, including private scraps and marked people;
• Support sharing media across networks.

Some features that might be interesting but are not included in my roadmap:

• Automate servers discovery using the WebFinger host-meta endpoint;
• Support rich contents (Noosfero articles);
• Paginate OrderedCollections;
• Add support to marked_people on scraps back;
• Whitelist for external environments;
• Relay messages.