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.

Docs
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.. toctree::
   :maxdepth: 2

   docs/features
   docs/installation
   docs/upgrading
   docs/FAQ
   docs/user_guide
   docs/customizing
   docs/admin_guide
   docs/xmlrpc
   Design (original) <docs/design>
   docs/developers
   docs/tracker_templates
   docs/acknowledgements
   docs/license