This will likely have to be reverted, to make things like $this->app->extend() work
reasonably well in extensions' service providers.
For now, since we fetch the enabled extensions from the config, there is no other way
for us to guarantee that the config is already available.
For now I’ve chucked it on Flarum\Core as a static method, but
ultimately I think we will need a ConfigRepository abstraction (whether
it replaces or sits underneath the Flarum\Core static method I’m not
sure).
Also starting to think about multisite scenarios, I think this is
important. The Forum model could actually end up with a database table
behind it, and each forum would have its own config settings? Haven’t
really thought about it too hard yet…
