Add support to load templates from json files

[bartvanhalder]

Our config.inc.php was getting a bit long and unreadable, so i added support for loading templates from .json files.

For existing installs this changes nothing as templates loaded from files are appended to the existing $templates before the "is this user allowed to use this template" check.

If an error is encountered loading a template a message is logged and the template is skipped.