Add a way to disable the canonical Tag in HTML output

[thomaszbz]

MetaSEO version: 2.1.0, 3.0.0

It seems as if the canonical tag can't yet be disabled for the case that the page properties contain a canonical entry.

There should be a new constant for this, and we should check this constant in https://github.com/webdevops/TYPO3-metaseo/blob/master/Classes/Page/Part/MetatagPart.php#L211

[thomaszbz]

Current plan is to change constant canonicalUrl to canonicalUrl.generateUrl. Then we can use canonicalUrlto enable/disable the tag as a whole (no matter if a canonical tag is set in page properties) in line 211.

For a very limited number of users, this is a breaking change because in TS, they must change usages of canonicalUrl to canonicalUrl.generateUrl

@tpfender Patches welcome