Cannot use markdown keyboard shortcuts when editing a message

Summary

When editing a message, you cannot use keyboard shortcuts for markdown (Ctrl+I/B/U, etc). For comparison, when writing a message in the message box, you can select words and use the shortcuts to add the appropriate markdown characters.

Steps to reproduce

1. Write a message containing at least one word. Note that when selecting parts of the message, Ctrl+B applies/removes the bold markdown characters **word**
2. Begin editing the message.
3. Select the word and press Ctrl+B.
4. Note that unlike when initially writing the message, Ctrl+B does not apply/remove markdown characters.