Query for venues on admin only

[trescube]

Fixes pelias/pelias#564

• adds predicate for checking whether request has focus point
• adds predicate for checking whether a specified layer was requested
• adds predicate for checking whether analysis was single field
• adds controller that appends results rather than replacing
• adds query for venues-only
• updates sorting module for ranking venues higher than neighbourhoods

[orangejulius]

This looks good to me and seems to be working pretty well on dev. What's left before we can merge?

[trescube]

Nothing left to do! Going to attempt to merge now (or soon, depending on how bad the rebase is).