By default Magnolia ships with an indexing configuration which excludes all so-called "system" properties. However, at least one of such properties, mgnl:tags, needs to be indexed and searchable. 

This is required by Periscope since we want customers to search by image tag in the search bar.

PR at https://git.magnolia-cms.com/projects/PLATFORM/repos/main/pull-requests/592/overview