Finish implementation of the admin UI (Fixes #479)

UI for executing queries:
- Real-time parsing/planning of the query to a MongoDB execution plan.
- Display results in a table, flattening all keys/indices and displaying
each unique path in its own column, and loading rows lazily.
- Save results as CSV to a file or the clipboard.
- When more than one mount is present, a drop-down appears to allow
selecting one of them.

New, Retina-compatible logo.

Many other small improvements.