WebSearch: new authorcount indexing and searching

• Introduces new 'authorcount' index containing number of authors in the record.

• Amends search engine in order to allow numerical queries like 'authorcount:12', 'authorcount:50+', or 'authorcount:9->12'.

• Introduces 'ac' SPIRES search operator that expands to 'authorcount', allowing queries like 'find ac 12', 'find 'ac 50+', or 'find ac 9->12'. (But does not allow 'find ac < 3' syntax; one has to write 'find ac 0->3'.)

• Adds corresponding regression tests. (closes #686)