This adds a top banner (and an unused alternative version), favicon, and
configuration file. There seems to be some automagic in place: if
`logo_path` is specified in config given to `catalog_create`,
DataLad-catalog looks for the file in `$workdir/assets`, then copies it
into catalog's artwork directory, and changes the config accordingly.
The favicon needs to be replaced after catalog creation.
Closes#2
Co-authored-by: Stephan Heunis <s.heunis@fz-juelich.de>