Externalize branding

@pblain As per your request.

The things which can be externalized are:
 * Logo
 * External links
 * Header ("Open Access to Ocean Data")
 * Landing page

This can be done by referencing a `brandingBase`:
```
portal.brandingBase = "http://somesite.org.au/branding"
````

Under `http://somesite.org.au/branding` you should provide:
 * http://somesite.org.au/branding/logo.png - PNG with the logo you'd like
 * http://somesite.org.au/branding/siteHeader - Header in plain text, such as "Open Access to Ocean Data"
 * http://somesite.org.au/branding/externalLinks.html - External links for lower toolbar in HTML format such as:
```
<a title="e-Marine Information Infrastructure" target="_blank" href="http://www.emii.org.au">eMII</a>  <b>|</b>
<a title="Australian Ocean Data Network" target="_blank" href="http://imos.org.au/aodn.html">AODN</a>
```
 * http://somesite.org.au/branding/landing.html - A custom landing page
Rather than having a special value 'landing' to indicate when the built-in landing page should be used can we just return null to indicate that?