Add default LTI settings to defaults.config?

[dlglin]

This is motivated by https://forums.openwebwork.org/mod/forum/discuss.php?d=8746.

When new config variables are added to any of the *.conf.dist files, they won't get set for anyone who is carrying forward config from a previous version while upgrading. This isn't a problem for most config variables since they should have a default value in defaults.config, but that's not true of variables that only apply to optional modules.

I have two thoughts that I'm wondering if they are good ideas:

1. For optional modules add any variables that have a default value to defaults.config. This has the benefit that if optional modules are extended and more config is required, then it should work for people with existing config files for those modules. The downside is that it means loading a lot of configuration that will never be used. We could do this for only the most commonly used modules rather than loading everything.
2. Set up each optional module with the same structure as the main config. i.e. have a module-name-defaults.config file that ends with include("conf/module-name.conf"), where module-name.conf would be built from module-name.conf.dist. Any site-specific changes would be made in module-name.conf, which wouldn't be affected by upgrades. This would allow users to upgrade to a new version while maintaining their module-specific config files from the previous version, and still have new functionality enabled.