Add education sitemap

Thom Wiggers
2019-06-06T15:29:33+02:00

education sitemap was not included in the sitemaps (the coverage of that file was 0%).

Also adding merchandise items fixture to the sitemap because that should help test if it generates things correctly.

#### Previous behaviour
Steps to reproduce:

1. curl https://thalia.nu/sitemap.xml | grep education

#### New behaviour
Steps to validate that it works:

1. curl https://thalia.nu/sitemap.xml | grep education