Remove themes, legacy config options and fix config bugs

[josebolos]

This is just a new branch created from master with all the changes from https://github.com/springernature/shunter/pull/195 applied, as that request is from 3 years ago so it doesn't merge cleanly. All the merit goes to @andrewmee.

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• Removes the following legacy configuration options which are no longer required:

  • config.path.themes - vestigial feature from when a single shunter instance would run multiple apps ("themes"). Now always set to the root directory of the host app, so no longer needed
  • config.path.shunterResources - Shunter no longer contains any default resources
  • config.path.templates - unused

• Allows custom configuration options to be passed to the Shunter testhelper for testing templates.

• Fix bug whereby Shunter would break if a user set a value for config.structure.templates other than the default view.

This pull request is a work in progress - opening now so I can document changes as I go.

TODO:

• consider changing behaviour and documentation around module use and local.json (no reason for this to be done any differently to setting other custom config - might be worth clarifying using the local json file is just an alternative method for setting all config).
• test-client.js provides no way to change default config, so will fail if config.structure.resources is changed from default (or any other paths referenced in that script)

[jpw]

This has a few changes which make me nervous. Unless @moddular is confident we can merge, Jose I think we should probably pair up and go through this in some detail next week.

[jpw]

I think this is good to merge, given we intend to remove Themes and also test-client.js in shunter v5.