Canvis a "Custom splash screen"

Ref. PWA08

NOTE: this feature is from a big development. See PWA discussion at GitHub.

Is your feature request related to a problem? Please describe.
As a visitor when I visit a PWA website with the Home Icon from “Add to Home screen” I don’t want to see a loading white page.

Describe the solution you’d like
For solving this issue, there’s the possibility of PWAs to configure the Splash Screen on the manifest.json file.

Describe alternatives you’ve considered
To have a native mobile app, but that’d be too expensive to implement.

Additional context
In the case of Decidim, it’s pretty important that this works with multitenant. It should also be configured on the admin panel (Appearance section).

https://web.dev/splash-screen/

Does this issue could impact on users private data?
No

Acceptance criteria

• Given that I’m an administrator
  When I go to the Organization -> Appearance section on the admin panel
  Then I and configure a new splash screen
• Given that I’m a visitor with the Home Icon
  When I click it
  Then I see the custom splash screen.