Geo: "Admin > Geo" page controls have to be thought out better

While working on https://gitlab.com/gitlab-org/gitlab-ee/issues/8112 I got some questions that have to be answered. This is a meta issue!

Disabling a secondary node

Why do we even need a way to disable Geo Node? According to what I see in the code, it just stops all kind off sync workers. We need to remove it or to document.

Removing a node

Again, we need to document it at least. Removing a primary node has a special meaning, we essentially make all the Geo nodes invalid. So removing Primary node without removing all the secondary nodes makes no sense at all. I propose to take few actions here:

• Hide "Remove" button in the primary node section until there is at least one secondary node in the list
• Mirror same behavior in API
• After removing the primary node we need to truncate event log table synchronously (this is very fast) and disable Geo as such. https://gitlab.com/gitlab-org/gitlab-ee/issues/8034

cc @geo-team