Add a 404 page matching akvo.org's

[Lynngre]

Update the 404 page to look like Akvo.org's.

[adriancollier]

I think it's important that we have a standardised feeling to the page, but I don't recommend that we have exactly the same 404 page as Akvo.org.

We have done lots of work in v3 to give RSR it's own distinct branding as a product - and I would like to continue this throughout the way it works. It should be as clear to users if they are on RSR or Akvo.org as it is clear when they are currently using FLOW.

[Lynngre]

Sure totally understand that :-) Your own funky 404 is good enough!

[damienallen]

Test plan:

Open invalid URL and compared 404 page to those on the main Akvo website.