Formatting issue on /me chat command when username mentioned

When mentioning another user inside the `/me` shortcut in a chat conversation, if the username is the last thing in the string, the formatting will break and the username will not correctly link to the mentioned user.

For example: `/me waves at @username` will incorrectly output as "\_waves at `@username_`" (notice the `_` Markdown syntax is output to the chat history and the last underscore is appended to the username).

Expected behaviour is for `/me waves at @username` to output "_waves at `@username`_".

This issue doesn't happen when the username is mentioned elsewhere in the string. For instance, `/me waves at @username to say hi` correctly outputs "_waves at `@username` to say hi_".

Tested using the macOS Keybase client version 5.1.1-20191211223249+15bbb94c23.

Start by reproducing the `/me` chat command in the macOS Keybase client, using a username as the final token and comparing it with a username followed by more text. Trace the chat formatting path and its mention handling. Done means the final mention remains linked and the surrounding Markdown underscores are placed as `_waves at @username_`.

Written by the indexing model from the issue text.