Alias 'impossibly thick thighs' to 'impossibly large thighs'

impossibly thick thighs -> impossibly large thighs
 
Same idea, different name. Impossibly large thighs is the older and larger tag, and already has a description and it’s necessary implications set up.

I would personally just change all 41 over manually.

@Background Pony #905E  
If it doesn’t get aliased, then it will just keep coming back.
 
Personally, I’d consider any tag over 20 uses past the point where an individual user should take it upon themselves to personally erase a tag.

Done.