When a field is added to a document without adding a configuration in the schema, the default behaviour is to index it, but not "store" its value. Therefore the addition of `lastModified` here would let us search on the property, but when we query the documents we don't get it back. In the UI, the lastmodified querystring variable in the URL is coming up as `undefined` in searches as a result of this.

Since `lastModified` isn't a standard schema property and we don't need to index it, we can just add it to `_extra`, so that it is stored, but not indexed.

I've made this change locally to verify that this is what was causing undefined lastModified values, so I'll just push that fix after merge if testing checks out otherwise fine.