Add configurable, persistent links to the header

[tiffon]

A site administrator should be able to add global, persistent links to the header.

Current needs for the links are:

• Support one level of grouping, e.g. Help -> { Wiki, Create Ticket, FAQs }
• Each link should have a label and a URL

The configuration mechanism should not be specific to this task, but general enough to support other scenarios, such as modifying the defaults for the search panel, etc.

Design of the configuration mechanism is TBD — details to be added after a design discussion.

[tiffon]

Configuration will be loaded from /api/config and can contain arbitrary data. Initially will have menu customization information.