Previous code used custom regular expressions to extract URLs, hashtags,
and mentions from text while the user was writing a post. These were
inconsistent with the ones that Mastodon uses so the derived character
count could be wrong.
As well as being visually incorrect this could prevent the user from
posting a status that was within the length limit, or allow them to
attempt to post a status that was over the length limit (which would
then fail).
Fix this by dropping the homegrown regular expressions and using the
same text parsing library that Mastodon users; twitter-text. This has
been converted to Kotlin and the functionality related to Twitter
specific features has been removed.
The hashtag handling has been adjusted, as Mastodon is more permissive
about the positions where hashtags can appear than Twitter is, in
particular, a hashtag does not need to be preceded with whitespace if
the tag appears after some scripts, such as Hirigana.
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