Tag alias: seashell_bikini

Aliasing seashell_bikini to shell_bikini
Reason: They're the same thing, really. Also, while the seashell_bikini tag hasn't been used/created, there is a possibility that someone would use that tag instead of shell_bikini. I almost did, myself.

The number of aliases we "could" create because someone "might" use them are nearly infinite.

I'll just add this one for now, but in the future I don't think such aliases should be suggested without a very specific reason.