externalized logo and landing page
externalized externalLinks
verifying branded redirects before using them
redirect landing page to externalized one if necessary
js taking external links from url via ajax
taking site header from branding or config file
portal branding refactored

 * refactored url fetching into neater functions
 * all values taken from config
 * builtin landing page decision made via null/not null variable
rename externalLinksHtml -> footerContent
tests for PortalBranding
Merge pull request #1375 from aodn/externalize_branding

Externalize branding