Model Configuration QoL & Usability Updates

[blackwire]

Adds several quality of life improvements and makes the application more easily understood.

• Sorts variables in the select-boxes by name so it's easier to select variables in this new format.
• Auto-selects the variable source if only one source is being used.
• Resolves an issue with error messages being somewhat vague and adds errors for variables within the variables box so it's easy to see the requirements.
• Existing variables (on an already configured model) are sorted in the UI by role then source then name so everything is grouped together nicely and a bit easier to find what you're looking for if you have many variables specified.
• Additional hint added when the feature role is selected to help with confusion around how these are used.