WebSearch: configurable SPIRES query syntax
- Adds variable CFG_WEBSEARCH_SPIRES_SYNTAX to configure the use of SPIRES search syntax. Amends SQPP to use the new variable in deciding if the SPIRES query parser is applicable. (closes #892)
- Amends unit and regression tests for SPIRES syntax, to be executed only if CFG_WEBSEARCH_SPIRES_SYNTAX is set accordingly.