Celtic mythology alias

BUR #52363 has been approved by @nonamethanks.

create alias irish_mythology -> celtic_mythology

These 2 tags are interchangeable, there’s only one picture under Celtic mythology that wouldn’t fall under Irish (This one post #7115501 based on a Welsh legend), so the Irish tag should be merged into Celtic.

There's less than a page of results of these, so we don't need a distinction between irish and welsh mythology at the moment. Might be worth to reconsider it if we get an influx of uploads, but for now this is fine.

The bulk update request #52363 (forum #407840) has been approved by @nonamethanks.