Document and include historical terms in AS2 context

[evanp]

"[T]erms like Hashtag, sensitive, manuallyApprovesFollowers, and movedTo, which were loosely defined in 2017/2018 but never formally adopted. Consequently, everyone just inlined them as context extensions and we all pretend that they're part of the activitystreams namespace and context despite not actually being in there."

"Can we work on defining some or all of them formally and officially within the SocialCG, then adding them to the normative context?"

[evanp]

@trwnh added this in #540, and I split it out as a separate issue.

[evanp]

I think the best process would be to wait for #540 to be complete, then to start working on them as an extension.