Replace popup with item info panel

[jimsafley]

As discussed in #7, instead of a marker popup containing only title and thumbnail, we should display a scrollable info panel on top of the map that contains more information about the item. This way the user won't have to navigate away from the map to get detailed information about the item. Of course we'd also provide a link to the item show page.

Info panel contents:

• Title
• Description
• Date
• Source
• Thumbnail
• Link to item show page
• [anything else?]