Allow disabling highlighting in SOAP search results

SOAP search results contain excerpts that are generated by finding a section that contains the search terms used for each matching document . Generation of these excerpts can become really slow on large instances.

Administrators should be able to change excerpts to only contain the beginning of the matching content as that is lot faster. This should be achieved by adding a -Dconfluence.soap.search.highlight.enable=false system property.