Remove the need for seoContentFields

[zanderwar]

Remove the need for seoContentFields() by using the DOM parser to better analyse the page content.

We have to consider that majority of the time there is always duplicate content across the site, which would be the header, nav and footer. We can assume that these tags are used correctly and great content is not placed within them, and then remove these entirely before the analysis.

I mean to get anymore accurate than that you would need cross-page analysis to determine the probability of what is duplicate content and what is not.

Currently, seoContentFields does not support many_many or has_many relations so may very-greatly reduce the accuracy of the current analysis.

[chillu]

Note that there's a related core RFC for this now: https://github.com/silverstripe/silverstripe-cms/issues/2454