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
I would suggest that the default value for all of these (site header, landing page, logo, external links) should come from Config. Currently some come from config (site header, external links) while other default values are defined here (logo url, landing page). Let me know if what I mean isn't clear.