[8.x] Expect custom markdown mailable themes to be in mail subdirectory

[jasonvarga]

Issue:

If you have a view named the same as your markdown theme, it will use that as the CSS file and cause an unstyled mailable.

This was introduced in 5f78c90 in order to implement #34391

Related: statamic/cms#2747

Steps To Reproduce:

• Keep your config/mail.php's theme as default
• Create a view named default.blade.php. (IMO a perfectly reasonable name for a view to have in an application)
• Send a mailable.
• See the mailable is unstyled.

The default.css won't be used because it'll try to use default.blade.php.

PR Proposal:

This PR attempts to solve both the original request in #34391 and this issue by expecting any custom themes to be located in a mail subdirectory.

Mark's request there looked to be like he basically wanted to be able to define mail themes without needing a vendor directory.

If you set theme to foo, instead of just looking for a view named foo it'll look for mail.foo.

If you set theme to mail.foo (like Mark would have done), it'll still just look for mail.foo and will be a non-breaking change.

This change is only considered breaking if you had a custom css file in the root or a subdirectory that wasn't named mail. That feels unlikely. Also, this feature was never documented.

[taylorotwell]

IMO the original PR should probably be reverted but we will roll with this for now and see how it goes.