Refactor of API timeline actions (#3263)

- Increase coverage to exercise all parts of each action
- Move into namespace to share common code
- Misc refactor of each action for smaller methods, simpler code

app/controllers/api/v1/timelines/base_controller.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+module Api::V1::Timelines
+  class BaseController < ApiController
+    respond_to :json
+    after_action :insert_pagination_headers, unless: -> { @statuses.empty? }
+
+    private
+
+    def cache_collection(raw)
+      super(raw, Status)
+    end
+
+    def pagination_params(core_params)
+      params.permit(:local, :limit).merge(core_params)
+    end
+
+    def insert_pagination_headers
+      set_pagination_headers(next_path, prev_path)
+    end
+
+    def next_path
+      raise 'Override in child controllers'
+    end
+
+    def prev_path
+      raise 'Override in child controllers'
+    end
+  end
+end