Fix problem when searching devices by contact names

The "type": "nested" prevented us from an working search when using just the contacts name as search term.

For us it mostly worked in the past on our local dev setup as we don't run the es reindex on startup. After that runs those searches didn't worked anymore.

After removing the nested type it seems to work again - and still supports all the other searches that we currently allow.

Closes #189 (closed)