Add `as:Hashtag` to activitypub context

[Tak]

At some point, mastodon got more strict about validating the context of incoming federated messages. Currently, it appears that hashtags don't federate correctly to mastodon when Note objects (and probably also Article, Person, etc.) contain a tag array with elements of type Hashtag, but don't define Hashtag in the @context.

I've verified that hashtags do not federate correctly (meaning: they're treated as normal links, posts with those hashtags don't show up in the hashtag search, etc.) from reading.taks.garden (bookwyrm 0.7.2) to either mastodon.gamedev.place (mastodon 4.2.8) or glitch.taks.garden (glitch-soc 4.3.0-alpha) before this change, and that they do federate correctly to both places after applying this change to reading.taks.garden.

Note: This only applies to objects that are federated "normally" (e.g. if a mastodon user follows a bookwyrm user), and you will not see the broken behavior if you "force" a note to be imported, e.g. by pasting a bookwyrm comment url into the mastodon search box

There was a previous discussion about this (over a year ago! 😱) at https://matrix.to/#/!zoxBMxLlvIyeEKkHuB:matrix.org/$1nDA0WY_s9NgaUFpki3GvX1yhV4KSF_8R1GFTwIQT4g?via=matrix.org&via=tchncs.de&via=mozilla.org

[mouse-reeve]

Oof! Thank you for investigating this