Redo the way we handle branding
With this merged, Ideascube now allows deployments to override the default branding simply by installing two files in the right place:
/usr/share/ideascube/static/branding/favicon.png
/usr/share/ideascube/static/branding/header-logo.png
As a consequence, all custom branding (Ideasbox and Koombook) we had inside of Ideascube is now removed, in favour of a new default branding. (see #302 (closed) and #466 (closed))
The branding might be extended in the future, so that more things can be overridden by deployments.