Change auto-following admin-selected accounts, show in recommendations (#16078)

app/models/account_suggestions/source.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+class AccountSuggestions::Source
+  def key
+    raise NotImplementedError
+  end
+
+  def get(_account, **kwargs)
+    raise NotImplementedError
+  end
+
+  def remove(_account, target_account_id)
+    raise NotImplementedError
+  end
+
+  protected
+
+  def as_ordered_suggestions(scope, ordered_list)
+    return [] if ordered_list.empty?
+
+    map = scope.index_by(&method(:to_ordered_list_key))
+
+    ordered_list.map { |ordered_list_key| map[ordered_list_key] }.compact.map do |account|
+      AccountSuggestions::Suggestion.new(
+        account: account,
+        source: key
+      )
+    end
+  end
+
+  def to_ordered_list_key(_account)
+    raise NotImplementedError
+  end
+end