Allow changing attribute type when creating it

When I add a new attribute to a team, I am able to select the attribute type:


However, if I select the wrong type by mistake, I am unable to change it, even though the attribute hasn't been created yet:


I realize that it's deliberate that an attribute's type is not editable after it's created; quoting https://github.com/developmentseed/osm-teams/issues/264#issue-1204400697 and https://github.com/developmentseed/osm-teams/pull/259#pullrequestreview-931623191:

> We should disallow modifying property type after it is created as there will be no way of parsing existing data (changing date to telephone, for e.g.)

...but in this case it should be safe because the attribute has not been actually created yet.

Start by locating the team attribute creation form and the attribute-type selector described in the issue. Verify the selector's behavior before saving versus after creation; done means a new, unsaved attribute can change type, while an existing attribute's type remains locked.

Written by the indexing model from the issue text.