Decouple the UX for adding primary vs. secondary Geo nodes

Per https://gitlab.com/gitlab-org/gitlab-ee/issues/4268#note_50616913

One other possible change to talk about: now that SSH sync is gone, there are no fields to fill in when adding a primary to the database via the UI. The URL must be the same as the instance that is doing the adding.

So, perhaps we can get rid of the "this is a primary node" checkbox entirely in favour of an "enable Geo" button (to be shown on the primary node only, perhaps part of the empty state?). When pressed, this creates the primary node.

The input form can then be unconditionally about adding new secondaries.

/cc @jramsay @hazelyang