Do not allow to register invalid teams for environments

[voidwisp]

Currently environment team invite page uses a different react component than the rest of the pages like environment linking and it allows to type any team name even if it is not a valid name and does not exist. It's easy to make a mistake and type a team that does not exist.

I propose we validate that a team actually exists before accepting it.

[dlpzx]

Hi @zsaltys this is a good enhancement and fits perfectly in the general purpose of 2.4: focus on UX and big-impact details