Extract duplicated edit logic into shared view

This view can be used for both the person and the organization editing
view so the logic isn't duplicated between these two very similar pages.
Fix bug with new organization button

The new organization button wasn't always working. This was because
there was some old code that was trying to open a modal to create the
organization in.

Instead it makes more sense just to link directly to the new entity
pages now (for both people and orgs) because they're no longer in
modals.
fixup! Extract duplicated edit logic into shared view
Make the delete membership button work again
Add placeholder text for membership date fields
Use an input rather than a select for organization