Move checking of alias to table function

This function is duplicated in both content history and tags and it makes more sense to move it to a common class (i.e. JTable). I've also added in a bit more content history stuff to ignore changes to certain fields.

My localhost on my laptop appears to be fooked so haven't been able to test this yet. So you might want to hold merging this until I fix it or can set up something on my own hosting (unless you have an undying urge to test it yourself :P)