rename transgender -> gender_transitioning

BUR #11728 has been approved by @nonamethanks.

rename transgender -> gender_transitioning

Since this tag is a constant source of mistagging, let's move the correctly tagged posts to a name that's less ambiguous.

The meaning of the tag will stay the same (posts related to gender transitioning or implying it), but it won't be involved in the constant bickering and tag reverting because of Bridget & co.

Wouldn't an alias be better here?

Also, what do we do about the people who mistake this change as another genderswap tag?

It's a rename so that transgender can stay empty until we figure out what we want to do with it.

I don't think there's a risk of people mistaking genderswap for this given its name.

The bulk update request #11728 (forum #219053) has been approved by @nonamethanks.