Advanced Search: standardize labels (singular, no question mark)

CRM/Activity/BAO/Query.php

CRM/Contact/Form/Search/Criteria.php

CRM/Mailing/BAO/Query.php

templates/CRM/Activity/Form/Search.tpl

templates/CRM/Activity/Form/Search/Common.tpl

templates/CRM/Campaign/Form/Search/Common.tpl

templates/CRM/Contact/Form/Search/Criteria/Fields/group.tpl

...tes/CRM/Contact/Form/Search/Criteria/Fields/sort_name.tpl