Allow user to user dropdown/column interface when viewing a capture

[machawk1]

When viewing a memento, the only options for navigation are to go back to the live web or iterative navigate one-by-one through the next/prev/first/last. Navigating to a memento two positions away (assuming not the first or last) thereby requires the user to visit a capture they may not care about.

It would be useful to allow the user to revert to the original interface with a dropdown/Miller column.

[machawk1]

This would entail surfacing the logic for creating the base Mink interface for selection from the "Viewing memento" interface. Given that the selection interface is built on activation of the extension, replicating the interface might requite more work or at least a tie-in to the current logic.