We mistakenly wrote to the key "hashtag" instead of "hashtags", resulting
in no results in the API response. Additionally, the type of the Tag's `history`
needs to be a list, not a dict.
This fixes hashtag search in Elk.
* Lowercase hashtag before loading its timeline
* Implement /api/v1/tags/<hashtag> endpoint
* Lower hashtag before un-/following
* Fix field name for hashtag following/followed boolean
* Various Toot! fixes
* Use a fallback 1x1 pixel image for missing headers, same as Mastodon. The header and header_static are _not_ optional nor nullable according to the spec.
* Try removing some fields which probably shouldn't be set.
* Pagination with next/prev.