Allow for custom icons and colour schemes

[zkamvar]

Right now we have a set of four colour schemes and logos based on our curricula. Of course, there are other organisations that use our infrastructure and would probably like to not have to create a new fork of varnish in order to use a different logo or colour scheme. Thus:

• [ ] create yaml item that can point to a logo/favicon file/s to replace the existing logo/favicon.
• [ ] create yaml item that can override some of the colorschemes (this may require modification of the scss directly, which would mean that varnish needs this as well)