Update fork to latest official master

Adding a "system_name" field to the "shotgun" descriptor allows for overriding the default behaviour in the "get_system_name" method which returns the wrong name for use in hook templates where the {engine} tag is used. 
eg, if a descriptor points to a toolkit bundle names "tk-houdini_release_v1.2.3", and a hook path is of the form "path: /some/path/to/{engine}/hook" then the {engine} field will be replaced by {entity_type}_{entity_id}, which will not point to the expected location of, for example, 'tl-houdini/hook' it will instead point to 'customNonProjectEntity01_104/hook'.
Adding an optional field to allow the user to set the "system_name" manually allows the default behaviour to be overridden where required without breaking existing expected behaviour.
This should resolve the issues raised on the community site on pages :- 
- https://community.shotgunsoftware.com/t/possible-hook-path-bug/3365
- https://community.shotgunsoftware.com/t/names-for-apps-frameworks-specified-via-shotgun-descriptors/2989

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