Breaking out the metadata pane into its own widget.
Adding a UI for adding, editing, and deleting metadata.

This included add, edit, and remove functions in the model as well as some
widgetry and templates for editing, creating, and deleting from the metadata
panel.
Adding access control checks for showing buttons and fixing edit.

Before, the edit was creating new keys in the event of a key change.
Handling some error conditions and squashing duplicate events.
Fixing JS style issues.
Always show the metadata pane, stop events, and update client-side meta.
Removing commented-out attempts at event squashing.
Fixing some issues with validation of metadata edits.
Merge pull request #170 from girder/metadata-edit

Metadata editing UI