Allow hiring page to be set in the admin

The hiring page URL was hard coded for speed. This should be an option in the admin.

This probably should be all set in a single file in `inc` folder, so I'd move the function for the redirect into there too. We'd probably also want this to be disableable, so we should add theme support.

Related: https://github.com/humanmade/hm-handbook-theme/issues/12 https://github.com/humanmade/hm-handbook-theme/pull/72

Start by locating the hard-coded hiring-page redirect and reviewing the `inc` folder, then compare the requirements with related issue 12 and pull request 72. Done means the hiring URL is configurable in the admin, the redirect can be moved into the appropriate `inc` file, and the behavior can be disabled through theme support.

Written by the indexing model from the issue text.