Add styling and branding to error pages & maintenance page

[walter]

Basically all the html files under public:

https://github.com/opengovernment/askthem/tree/master/public

As well as https://github.com/opengovernment/askthem/blob/master/app/views/layouts/maintenance.html.erb.

As mentioned in #195, these should probably not rely on assets stored locally in the application, but in S3. Coordinate w/ @rchekaluk.

[acacheung]

super simple