Send an email to authors when their content is flagged by a user

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

When I create content in Decidim and someone flags the content to be moderated, I want to be notified about it.

This is related to !15627 (https://meta.decidim.org/processes/roadmap/f/122/proposals/15627)

**Describe the solution you'd like**

When this happens, I want to receive a notification/email about this. The notification will not include the user who reported the content, only the reason.

**Describe alternatives you've considered**

None.

**Additional context**

None.

**Does this issue could impact on users private data?**

No.

**Funded by**

EU Comission