Use ActivityPub on publication side

[acka47]

This is related to #37. We've already discussed this a few times.

Currently, SkoHub only uses ActivityPub when sending out notifications to the followers of a SKOS concept. When people publish resources to the concept, we only implement the broader Linked Data Notifications spec, thus allowing any (?) JSON-LD document.

Here is how I imagine a ActivityPub-based process for publishing to a concept's inbox using SkoHub Editor:

1. When opening up the editor or extension I am asked to log in with an ActivityPub/Mastodon user, similar to a Mastodon web client like Pinafore.
2. I describe an online resource and add subjects
3. When clicking "publish" the JSON-LD metadata is wrapped in a Note (or Create?) activity which is sent to the concept's inbox as well as (optionally) to my followers.

This approach enables a "boost" of the note by the concept to its followers (Announce action) retaining information and links to the original author.