Change colors on mobile navigation bar

Ref. PWA07

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, it’d be nice if Decidim gets a better integration on mobile browsers (Android, iOS, and Window Phones).

Describe the solution you’d like
For doing this there are some HTML meta tags that gets used:

<meta name="theme-color" content="#4285f4">

We should also investigate with other mobile platforms and other kinds of tags. It’s important that this is also compatible with other configurations of decidim and works in Multitenant instances, so this should be set at Organization -> Appearance.

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

Additional context

Resources:

• https://web.dev/themed-omnibox/
• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name/theme-color
• https://developers.google.com/web/fundamentals/design-and-ux/browser-customization

Does this issue could impact on users private data?
No, it’s about colors.

Acceptance criteria

• Given that I’m an administrator
  When I go to Organization -> Appearance I have a new setting for theme-color and I can change it to #56a832
• Given that I’m a visitor with a compatible browser and this color was changed
  When I go to a page of this instance I can see that the color of the header of my browser is green (#56a832)