Activitypub/Add `discoverable` parameter in ActivityPub actor

See also pleroma-fe!947 (merged)

This MR is a subset of !1616 (closed) which focuses only the discoverable parameter.

This MR suggests to add the parameter discoverable in the ActivityPub actor.

The discoverable parameter is the opt-in signal which allows to show a user in the Mastodon's profile directory. Pleroma has not profile directory yet, but Mastodon's profile directory is showing federated (external) users. If a Pleroma user sends discoverable: true via ActivityPub, the user is shown in some Mastodon instance's profile directory.

User Matching for Fediverse is also use this parameter for opt-outing.