Support rich text formatting

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Before marking this merge request as Ready, the following must be done:

在把这个合并请求标记成就绪之前，这些东西必须完成：

• The code must compile. Preferably, each commit should compile.

  代码必须能编译。最好每个提交都能编译。

• [n/a] Any added UI text must be translated via l10n.get(). The messages must be translated into every core language: en, cmn.

  任何添加的 UI 文字都要用 l10n.get() 翻译。 讯息必须翻译成所有核心语言：en，cmn。

  • [n/a] The merge request creator must translate each message into one of the core languages. The others can be handled by others.

    开合并请求的人必须把每条讯息翻译成一种核心语言。 别的可以给别人做。

• If the merge request contains a user-visible change, it must have a changelog.

  如果这个合并请求包括对用户可见的更改，必须写个更改记录。

added 1 commit

• 36947ccd - Remove quote in quote when replying

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added 1 commit

• 10909cd6 - Make run with cmark 0.29

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added 1 commit

• 0bbf2a07 - Make run with cmark 0.29

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added 1 commit

• 5d651f7a - Make run with cmark 0.29

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added 7 commits

• 5d651f7a...549f2f6a - 2 commits from branch servant
• 35875af3 - Display rich text in event
• 94f9dc65 - Support sending markdown-formatted messages
• 24692780 - Add change log for rich text formatting
• de41f0f4 - Remove quote in quote when replying
• ef00d49f - Make run with cmark 0.29

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merged