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

should this snippet be "post_header"?