`portfolios.create_member` now just sends an invitation, so it should be
with the invitation routes. This also de-duplicates the function for
sending a portfolio invitation email.
In order to place modal forms in other places on the page (so that forms
are not nested) it's necessary to move MultiStepModalForm links out of
the component. They just need to refer to the correct modal.
This PR also makes changes to ensure that the active modal is being
unset everywhere correctly when a modal is closed.
