Adding secure routes for `pre_header` and `post_header` 

> Trello: https://trello.com/c/9K65rlW5

## Narrative
We are in the process of making a responsive checkout path for the Shop. These layout snippets are needed for the effort.

## Verification Tasks
1. Verify that `http://localhost:8080/secure/layouts/responsive/pre_header` responds with the pre header snippet
1. Verify that `http://localhost:8080/secure/layouts/responsive/post_header` responds with the post header snippet

> **Notes:** I didn't find any corresponding routes in Dispatch for these secure paths, but if there's any other configuration needed for these to work, please point me in right direction