Allow changing attribute type when creating it

[waldyrious]

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 #264 (comment) and #259 (review):

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.