MLPF's tags are all lowercase now

[Twilight Sparkle ✨ 8,520]

I'm cleaning house with MLPF's tagging system and, as part of that, converted all extant tags to lowercase. Going forward, all new tags will automatically be converted to lowercase, too.

Why do this? It makes tags all over the site more consistent. And, given the system is already rather freeform, gives us all one less thing to think about getting right when creating new tags.