Improve UI for categorical facet Select all/Select none

[msmolens]

An idea from https://github.com/ImageMarkup/isic-archive/pull/383#issuecomment-290807439:

Mimic all or part what http://kayak.com does when searching for flights:

• Provide an "only" link, which appears on hover, to select only a single item.
• Only whenever not all items are selected, provide a "Show all" link. The link is placed next to the section header so it's clear that it's not just another checkbox.

This would optimize the workflow of picking "Select none" followed by selecting an item. Screenshots for reference: