[Example] Assign route to custom entity

[niklasnatter]

What's in this PR?

This PR demonstrates how to add a route to a custom entity using the route content type and the RouteBundle. This PR builds upon #84.

To apply the changes, the schema of the database needs to be updated by executing bin/console doctrine:schema:update --force.

[joelbeckmann]

Thank you for this Example! I have encountered one problem for which I have not found a solution yet: When you are in the event editor and click on the button that shows you the URL history, I only get shown the history for the locale that is currently selected in the webspace. I think that behaviour can lead to some confusion for the end user. Does someone know if there is a way to show the history for all locales in the route editor?