Add custom favicon option

[meericsc]

Add custom favicon option to theme to make it possible to customize. Maybe also robots.txt? It could be that changing the order of resource service (extra + theme before resource) could solve robots.txt but favicon could be better as explicit. Maybe document how to do both of these?

[Macroz]

It's a good idea to handle these. It's also possible to do these in some proxy that REMS sits behind. And I think this was used a long time ago in practice.