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
Currently every resources is requested twice. Once to see if it will work and then once again to get the content. We could optimise that so each resource is only requested once.