The purpose of this button is to copy a message source, including the markdown. The solution uses a recommended approach https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Interact_with_the_clipboard of interacting with the clipboard.

Linear task: https://linear.app/comm/issue/ENG-2332/copy-message-button-on-web

In this diff we're not giving any feedback to the user, especially we don't display a different tooltip when a message is successfully copied. https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A78271&t=LDQhPB76aimwoBhD-0 This will be handled later in the stack.

Depends on D6013

1. Check if the button is present only for text messages and sidebar sources based on text messages.
2. Check if copying a text messages with non-markdown content results in clipboard being filled with its content.
3. Check if the same happens when a message contains some markdown. Paste it and send to verify that a new message looks the same.
4. The testing was performed on Chrome, Safari and in Electron.