Arena/Stadium tags reorganization

BUR #26250 is pending approval.

create alias arena -> stadium
create implication colosseum -> stadium
create alias baseball_stadium -> stadium

The tags Arena, Stadium, and Colosseum were all very similar tags that were almost always mistagged with one another.

It seems like the tag "arena" was more used to the more "fantasy"-looking stadium, while in reality the name is specifically (albeit often interchangeably) used for an enclosed stadium. While the tag "colosseum" is more often than not applied to any generic "Roman-esque" stadium rather than the actual real life stadium.

Given how the "arena" and "stadium" tags were functionally redundant, I think these two tags should be merged into one to prevent further conversion. While I think the tag "colosseum" would need quite a lot of cleanup to distinct the actual real life colosseum with the fantasy ones. Also I supposed we merge the tag baseball stadium into the stadium tag, since I don't know why it should warrant a separate tag when other more popular sports like football doesn't need their own specific tag for their stadium.

I have no strong opinion on folding arena into stadium, if the mistags are really that bad. I'd like to say that maybe it's worth giving the tags another chance by gardening them, but that's not always practical.

On the other hand, I disagree with folding baseball stadium into stadium, especially with the reasoning that we should do so because other sports don't have their own venue tags. Why not make them then? Soccer already has one for its field (soccer field) and ice hockey has one in skating rink. It wouldn't be difficult to just make a handful of tags for the sports that don't have them like american football.

I'm also opposed to it because a "baseball + stadium" search isn't a sufficient replacement for the tag. There are multiple posts taking place in baseball stadiums which don't actually show baseball being played. There are posts with uriko like post #6512969 or posts with fans cheering like post #6597938 that don't show the sport, and posts like post #7334260 or post #7084219 which contain baseball stadiums and don't even imply that the sport is being played, either because the person is out of place or because the stadium is empty, respectively. Basically, I think it's a bad idea to fold a distinct location like a baseball stadium into a more general tag for no good reason.