Search web ui

This is not ready to be merged yet, but I'm opening the PR so it can be reviewed and discussed with the UX team and tested on a server with reasonable search index.

 - Search form now resembles the mock-up in https://docs.google.com/document/d/1SzCSZH1IQOKiC_aBjPhe3bFWvzW06j1dlYFWPlyActQ/edit
 - Search is now cross-group by default but can be a specified group and user
 - By default we search on all types and fields, but these can be restricted.
 - Searching with the field in the webclient top-right toolbar uses the defaults (All types, fields, groups).

TODO: 
 - UI tweaks on form
 - Results: pagination? filtering?