Change followers page to relationships page in admin UI (#12927)

Allow browsing and filtering all relationships instead of just
followers, unify the codebase with the user-facing relationship
manager, add ability to see who the user invited

app/controllers/admin/relationships_controller.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Admin
+  class RelationshipsController < BaseController
+    before_action :set_account
+
+    PER_PAGE = 40
+
+    def index
+      authorize :account, :index?
+
+      @accounts = RelationshipFilter.new(@account, filter_params).results.page(params[:page]).per(PER_PAGE)
+    end
+
+    private
+
+    def set_account
+      @account = Account.find(params[:account_id])
+    end
+
+    def filter_params
+      params.slice(RelationshipFilter::KEYS).permit(RelationshipFilter::KEYS)
+    end
+  end
+end