Adding secure routes for `pre_header` and `post_header` layout snippets that are needed for the Lonely Planet Shop checkout path to be responsive.

Trello: https://trello.com/c/9K65rlW5
Updating post_header snippet property to correct value
Removing overly specific routes and adding a `secure/layouts/:route/:snippet` matching route
Merge pull request #977 from lonelyplanet/srb-secure-preheader

Adding secure routes for `pre_header` and `post_header`