Use RFC 2119 language for Group actor properties

[evanp]

"A group can be described like any other resource. The following terms from the ActivityStreams context (https://www.w3.org/ns/activitystreams) can be broadly useful:"

Let's change this into normative language:

• MUST have an id (AP)
• MUST have a type (AP)
• SHOULD have a name (AS2)
• MAY have summary (AS2)
• MAY have icon
• MAY have image
• MUST have outbox (AP)
• MUST have inbox (AP)
• SHOULD have attributedTo
• SHOULD have addressing (to, cc, bto, bcc, audience)
• SHOULD have published
• MAY have updated

Linking to the Activity Vocabulary and ActivityPub docs can provide the purpose and full URIs for these properties.

[TallTed]

s/RFC 3119/RFC 2119/