Refactor pages and news

[ferblape]

✌️ What does this PR do?

This PR implements partially #932

• It simplifies the URLs of pages and news to be:
  • /paginas/collection-slug
  • /pagina/page-slug
• It uses a template system depending on the collection container:
  • for site containers, pages use gobierto_cms/pages/templates/
  • for participation, pages use gobierto_participation/pages/templates/
  • for participation processes, pages use gobierto_participation/processes/pages/templates/

A lot of methods, templates and actions have been removed too.

🔍 How should this be manually tested?

Checking this issue will be hard. Here there are a few suggested steps:

• check a collection of pages with a Site as container
• check a collection of pages with Participation as container
• check a collection of pages with a participation process as container
• check a collection of news with a Site as container
• check a collection of news with Participation as container
• check a collection of news with a participation process as container
• check a site where a section is configured as the home page
• check the information page of a process
• check the news page of participation
• check the news page of a process

👀 Screenshots

Doesn't apply, it changes at URL level