When quoting a chat message in a post, if that message contains a mention,
that mention should be ignored. But we've been detecting them and sending
notifications to users. This PR fixes the problem. Since this fix is for
the chat plugin, I had to introduce a new API for plugins:
# We strip posts before detecting mentions, oneboxes, attachments etc.
# We strip those elements that shouldn't be detected. For example,
# a mention inside a quote should be ignored, so we strip it off.
# Using this API plugins can register their own post strippers.
def register_post_stripper(&block)
end
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