Move cross-table api counts into plugin

[ErisDS]

Another piece of refactoring work on the way to making it possible to add cross-table counts etc.

The key thing here, is that rather than having a util that is then called inside the pagination plugin and the tag.read function, and would reasonably have to be called at other points too, we now have a plugin that automatically gets called at the right times.

This makes it easier to add support for counts on other models / relationships. Although, I still need to figure out how to add the behaviour so that only the correct posts are counted (what I'm working on next...)

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One thing that has occurred to me is to use this fetch/fetchAll override pattern to create a query-building hook system into fetch/fetchAll and allow for the registering of functions to be called by that hook if certain options exist.

E.g.

• if there is a count property in an include, call count handling
• if there is an order property in the options, call order handling
• if there is an group property in the options, call group handling

I think this could work to further simplify our code for building queries.

refs #6009, #5615

• minimal refactor to remove the addition of count from pagination and other various points
• create a include count plugin that overrides fetch and fetchAll
• this ensures that counts get added at the right points