Show indicator of new message in top navbar

# Problem

From https://github.com/code-corps/code-corps-api/issues/1234#issuecomment-349187243

There should be an indicator of a new message in the page's top navbar. Clicking it takes the user to the user messages route implemented in #1593

It will use the user's `unread_conversations` counter cache column.

Blocked by https://github.com/code-corps/code-corps-api/issues/1316