Choose how to handle edge cases where the user has already an entity in the vocabulary but we find also an article/page/category/tag that might fit

[cyberandy]

Please describe the issue here:

As a general rule, we want to optimize traffic by leveraging on already existing content - If I have an entity in the vocabulary but we also find an existing article/page/category/tag that could match that entity we shall - if the user makes the match from the new UI (see #940) - add a canonical URL to the already existing article/page/category/tag.

This will instruct Google to consider - rather than the entity - the newly matched page. This is for advanced users only but it is nice to have it.