Implemented scaffolding for System Settings and implemented configurable Saving Throw names.

[DC23]

The basic approach is entirely data-driven

• The settings data object is built for the saving throws, with default values pulled from the localisation database
• The Actor sheet reads values from the system settings object instead of from localisation strings, using the same key as before.
• Default behaviour is identical: localised saving throw names identical to whatever is in the language files
• A GM-only settings menu allows the saving throw names to be overridden for the entire world in a small settings form. Form validation prevents empty strings. I apply simple string cleaning to strip whitespace from the start and end of the string.
• There is commented out code that uses a regex to strip HTML tags from the strings, but given that Foundry allows GM and players to execute arbitrary client-side JS code anyway, I don't see that allowing unsanitised strings as saving throw names is really a problem. If it is, then I'd suggest that rather than rolling our own dodgy regex, we'd import a JS library and call that.
• Settings menus like I've used give good control over validation, but unfortunately they mean we don't get the free "reset defaults" function. I implemented this manually. It's not hard and it works.
• I'm quite happy with the overall approach. Everything is data-driven and agnostic with respect to the number of string items being handled in the settings. I think there's a nice general pattern emerging that could be abstracted further if this needs to be repeated for other things.

[DC23]

Will fix #75 and #69