Remove gobierto dashboards

[entantoencuanto]

:v: What does this PR do?

Removes GobiertoDashboards and lets external gobierto engines to declare dynamically their own modules

This PR has some issues:

• Some tests are failing because the GobiertoDashboards::Dashboard model is not available from main app
• The fixtures on the gobierto-pro engine are not loaded in test enviroment, so the test introduced by it fail.

:mag: How should this be manually tested?

:eyes: Screenshots

Before this PR

After this PR

:shipit: Does this PR changes any configuration file?

• [ ] new environment variable in .env.example?
• [ ] new entry in config/application.yml?
• [ ] new entry in config/secrets.yml?

(Changes in these files might need to update the role in Ansible)

:book: Does this PR require updating the documentation?

• [ ] new site configuration variable?
• [ ] new site template?
• [ ] new module/submodule settings?
• [ ] significant changes in some feature?