Optimize First Contentful Paint

Ref. PWA02

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

Is your feature request related to a problem? Please describe.
Loading Decidim is pretty slow most of the time.

Describe the solution you’d like
As a visitor, I want that the time at which the first text or image is painted is less than 2 seconds.

Additional context
https://web.dev/first-contentful-paint

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

Acceptance criteria

• Given that I’m a Decidim developer
  When I go to the Lighthouse Workflow in GitHub Actions
  Then I see that the …
• … First Contentful Paint of the homepage is less than 2 seconds.
• … First Contentful Paint of a participatory process is less than 2 seconds.
• … First Contentful Paint of a proposal is less than 2 seconds.
• … First Contentful Paint of a meeting is less than 2 seconds.