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 (pressing start doesn't start :( ) 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)