Add the extend attribute to the theme info via the theme info comment
block rather than adding it in the volse_init() method. This also makes
the volse_init() method obsolete for now.
See MR 2217:
https://framagit.org/hubzilla/core/-/merge_requests/2217
Also add a theme config class that extends the redbasic config class.
This enables the extra theme configuration and schema support from
Redbasic.
