Improvements to synonyms

[ojwb]

fastcardriver on #xapian ran into this not working:

db.add_synonym( "Treppe", "Treppenkörper" );

The issue is the term needs to be lowercase (otherwise T is treated as a prefix), but we don't actually seem to explain that.

Also they noted that it seems wrong that add_synonym() is called from the search code. Perhaps it would be better to have separate scripts for indexing, adding synonyms, and searching to make it clear that these are essentially separate steps.

[jaylett]

Agree on both counts. It's weird that search_synonym has to use a writable database, where none of the other searches do; splitting out synonyms would allow us to resolve that.