/sit allows floating players

[Zomon333]

/sit allows players to sit on things such as signs, carpets, slabs, etc; partial blocks that don’t have collision. This allows players to do things such as jump and sit, and end up floating mid air because they are considered valid blocks to sit on.

Normally, a player can’t float and so this isn’t an intended use of this feature.

[14mRh4X0r]

See reasoning for #462