Allow adding existing teams to an organization

According to [the docs](https://mapping.team/guide#add-org-team), it is only possible to add a team to an organization at the moment that the team is being created:

> To add a team to an organization, check the box “This team belongs to an organization” **when creating a new team**.

(Emphasis mine).

It would be more convenient to allow adding an existing team to an organization, in case it wasn't created as such in the first place.

Start with the team-creation flow described in the linked add-org-team documentation and trace how organization membership is handled. Determine the existing team-management entry point, then verify that an already-created team can be associated with an organization and that the documented behavior matches the result.

Written by the indexing model from the issue text.