Result selection controls

[LanesGood]

Problem

Users want to be able to select a subset of n results from the entire body of returned results

Solution

Provide a mechanism for file/result selection.

Requirements

• [x] Each result card/table row includes a checkbox
• [x] Users can select individual or multiple items
• [x] Selected results persist across search results pages
• [x] #94
• [x] A counter indicates the number of results selected
• [x] Users may click "clear" to deselect all selected

UI Proposal

[geohacker]

Let's move this low priority for now.