WebSearch: fix various span query problems

• Fix various span query problems, notably (i) search guide span example query in the author index that stopped working after special author tokenizer got introduced; (ii) span query was triggered for any occurence of `->' causing problems when searching for reactions. (closes #268)

• Note: the span query is now triggered only when a space is not present immediately before or after the `->' operator. Such a escaping/triggering decision technique is not fully ideal or the most user-friendly ever, but it works without altering the current parsing stage technique and it respects the current search guide syntax documentation on span queries. We may want to solve the escaping issue later inside parser stages though.

• Add test cases for all these various span queries. (word/phrase/marc/author/escape)