Make the portal page more customizable

[gregcorbett]

• [ ] additional logos at the top, similar to what has been done for https://gocdb.eosc-portal.eu/portal/
• [x] #361
• [ ] option to hide "Participating National Grid Initiatives (NGI)", see https://gocdb.iris.ac.uk/portal/ (perhaps based on wether the NGI menu item is shown or not).
• [ ] option to show a link to the list of services, see https://gocdb.iris.ac.uk/portal/ and "For a list of services in the infrastructure, see here."

Extending the local view work to achieve these seems sensible, as does implementing them in a similar way to the current read only warning.

[gregcorbett]

As alternatives to hiding "Participating National Grid Initiatives (NGI)" and optionally adding "For a list of services in the infrastructure, see here.", maybe we just rework the text on the front page to be a bit more generic.

[gregcorbett]

I've seperated out the preproduction part of this into it's own issue: #361