An exception in the webapp was trown when an empty mention was sent.
Examples of problematic messages are "@" or "@****".
In order to fix this, the regex that identifies mentions has been
modified, so it now requires the mention to have a "content" (by
replacing the ? quantifier by +).
A test case has been added to `frontend_tests/node_tests/echo.js` to
check that this works properly in the future.
ca190bd394