OPDS: add Standard Ebooks server (#8606)
OPDS: add Standard Ebooks server (#8606)

Standard Ebooks is roughly a small subset of
Project Gutenberg with better formatting.
Frenzie/koreader,poire-z/koreader,poire-z/koreader,Frenzie/koreader,koreader/koreader,koreader/koreader,NiLuJe/koreader,NiLuJe/koreader