[slack] mention & text formatting

https://zapier.com/help/doc/tips-formatting-your-slack-messages

Tips for Formatting Slack Messages

Text

• Bold text: wrap your text with asterisks (*).
  *bold*
• Italic text: wrap your text with underscores (_).
  _italic_
• Strikethrough text: wrap your text with tildes (~).
  ~strikethrough~

Emoji

• Wrap the name of the emoji in colons (:).
  :heart_eyes:

Mentions and notifications

• User mentions: type an open angle bracket, the at symbol (@), the user’s Slack member ID (see below how to find the member ID), and a closing angle bracket.
  <@UG1234567>
• “Everyone” notification (notifies everyone in your workspace in the #general channel): type an open angle bracket, an exclamation point (!), “everyone”, and a closing angle bracket.
  <!everyone>
• “Here” notification (notifies active members in a specific channel): type an open angle bracket, an exclamation point (!), “here”, and a closing angle bracket.
  <!here>
• “User group” notification (notifies all members of a user group): type an open angle bracket, an exclamation point (!), type in “subteam” , a caret symbol (^), the user group ID (see below how to find the group ID), and a closing angle bracket.
  <!subteam^abcde123456>