[tag_history] show colour-coded tag changes, remove tag aliasing

[discomrade]

This makes it much easier to see when a tag was introduced in each entry.

Using explode instead of Tag::explode prevents tags being aliased in the tag history, which could make some entries identical (consider the example, where the aliases stirner->max_stirner and anarchist->anarchism were introduced. If Tag::explode were used, it would not show any change as the tags in the old entry would be aliased to be the same as the newer ones). This also prevents the database being queried on each tag, massively improving performance on tag history pages.