issue2550891: Allow subdir in template value. Anthony (antmail) requested the ability to put templates into subdirectories. So the issue class can accept @template=issues/item to get the html/issues/issue.item.html template. Inlcude a test case for missing and existing (tal) templates. Also include a test that fails path traversal check. Add mention of subdiectoy use to customizing.txt along with some spelling fixes and ^M removal.