Recently active hashtags

When the user types # on the Search screen field, display a list of hashtag suggestions. This list is styled similarly to the design shown in #2051.

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• ssb-threads: recentHashtags() returns a pull-stream of recently-published-on hashtags, sorted by recent-first.
  • Traverse all posts by recent-first, and then collect all the hashtags in that post

Past exploration

Tags (or hashtags) are magical words, portals to relevant content. User can read them like any word, but can also explore their affordances.