Add support for machine translations of user-generated content

**Is your feature request related to a problem? Please describe.**

When a user submits content from the public area, they don't have a choice to translate the content. And, if they did, maybe they didn't know all the languages of the organization, leaving other users unable to read that content. This happened in Metadecidim, where we started using Catalan and Spanish, and later added English. This made new users potentially unable to read some of the content of the website. **Describe the solution you'd like**

We propose adding a way to automatically translate user-submitted content. We'll work on a Decidim module to handle this feature, leaving integrations with real services up to other developers. This is the same idea we have with verifications: Decidim has the whole framework, but the real integration is made by each installation. Possible future integrations might include Google Translate or Bing, for example.

This should be both an installation configuration and an organization setting. The installation configuration is needed so that the app knows there's a way to translate content, and the organization setting will be used to enable this per organization.

Note that this proposal will affect submissions from any side of the app: public and admin. We'll avoid translating for a locale that has been submitted by a user, and users will be able to edit any machine-translated content if Decidim allows it (forms in the admin section, for example). **Describe alternatives you've considered**

None. **Additional context**

None. **Does this issue could impact on users private data?**

No.