Send email functions changed to use templates

Send email functions changed to use templates using new function getTemplateContent().
Bigger change (sql) in churchservice_openservice_rememberdays().
Content of functions informLeaderAboutNewGroupMember(), ...EditedGroupMember()and ...DeletedGroupMember()put into new function informLeaderAboutMembershipChange().

Several little changes, fixes (f.e. regarding user simulation), renaming, speaking constants.

Please review the FIXME: Tasks!