QueryBuilder: show on its own page, link from home

Created by: sqs

The query builder took up a lot of space on the home screen and had an unsightly link (that looked cluttered especially when there were search scopes, quick links, and/or notices). Now that we are introducing split search modes, we have a better place to link to the query builder: from the search mode menu.

To make the query builder link less cluttered for now (with splitSearchModes off), the link is slipped in beside the search scopes.

Includes other refactor and move commits.

New query builder page

Empty state

Link in search mode menu (when splitSearchModes is on)

Link in search scopes list (when splitSearchModes is off)