[10.x] Config markdown theme ignored

[AEM5299]

This is a bug introduced by my earlier PR #46200. It is always defaulting to default theme unless explicitly set in the Mailable class, which ignored the config option (reported by @pikseli). This should fix that bug.

I am not sure if this is the best approach, so open to any suggestions or other ideas.

[driesvints]

Thanks @AEM5299. But I'm afraid this PR can't be merged as it contains breaking changes to class constructs and replaces properties. I'll try to have a look myself.

[AEM5299]

We can get the value from config instead of always setting it to 'default' here. We can also change Markdown to not be a singleton, since it correctly sets the config value on construction.

[driesvints]

I think the former would work. You can maybe use Container::getInstance() to resolve the config value.

[u01jmg3]

@AEM5299: not sure if it's intentional but your PR no longer has any test coverage.

[taylorotwell]

Can you please fix conflicts?

[taylorotwell]

Fixed here: 791f8ea