Provide a ThemeServiceProvider for themes

[tabacitu]

WHY

BEFORE - What was wrong? What was happening before this PR?

Two things:

• each theme had its own AutomaticServiceProvider, with the same logic, and it was difficult to change logic in all of them all at once; more info in Move ThemeServiceProvider and AddonServiceProvider to CRUD #5147
• for themes, we weren't loading the views if the theme wasn't active - more info in Fix theme switcher demo#530 (comment)

AFTER - What is happening after this PR?

Backpack provides a ThemeServiceProvider and that loads the views even if the theme isn't active.

HOW

How did you achieve that, in technical terms?

• moved AutomaticServiceProvider from all themes to CRUD;
• renamed it to ThemeServiceProvider;
• turned it into a class (instead of a trait);
• made all AddonServiceProviders in themes extend this new ThemeServiceProvider;

Is it a breaking change?

Yes. Sort of. I mean it's a breaking change only if you're using a particular version of a theme without having the latest version of CRUD too. Which is something only our team members would do. So no, non-breaking for anybody testing Backpack, other than our team members.

[tabacitu]

After this PR is merged we should merge:

• use ThemeServiceProvider theme-coreuiv2#15
• use ThemeServiceProvider theme-tabler#86
• Use theme service provider theme-coreuiv4#25

[tabacitu]

@pxpm let me know if doing this sounds like a bad idea to you. No testing needed. Otherwise give me the green light, I'm gonna go ahead and merge. It's all tested and working great.