New Config class

[franzliedke]

This extracts another real class for dealing with the configuration options stored in config.php.

Similar to #2142, where I introduced a Paths class. The idea is to reduce the scope of the Application class and make it easier to inject exactly what's needed (rather than an array, which is complicated, or the bloated Application class).

[askvortsov1]

Overall I like these changes a lot, much nicer now that we can inject the config. Left a few small comments here and there

[luceos]

An unforeseen consequence of making the url required in the Config class is that the console install command no longer works. In fact nightly also is broken due to this change. If you can propose/recommend a solution I'm willing to patch core @franzliedke. Personally I am not sure, because making url optional in the Config reduces the need for that class altogether.

Maybe we should have different classes per environment, but that feels unnecessary as well.

What do you think?

[askvortsov1]

Perhaps we should just make it not required again? Or pass in an array of "required keys" to the constructor (with the default being ['url'])?

Either way, this is another reason why we need more robust e2e testing, so it's good to see us moving towards that.