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/public_controller.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module Api::V1::Timelines
+  class PublicController < BaseController
+    def show
+      @statuses = load_statuses
+    end
+
+    private
+
+    def load_statuses
+      cached_public_statuses.tap do |statuses|
+        set_maps(statuses)
+      end
+    end
+
+    def cached_public_statuses
+      cache_collection public_statuses
+    end
+
+    def public_statuses
+      public_timeline_statuses.paginate_by_max_id(
+        limit_param(DEFAULT_STATUSES_LIMIT),
+        params[:max_id],
+        params[:since_id]
+      )
+    end
+
+    def public_timeline_statuses
+      Status.as_public_timeline(current_account, params[:local])
+    end
+
+    def next_path
+      api_v1_timelines_public_url pagination_params(max_id: @statuses.last.id)
+    end
+
+    def prev_path
+      api_v1_timelines_public_url pagination_params(since_id: @statuses.first.id)
+    end
+  end
+end