Attachments to accountability results

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

When a project moves to the accountability phase and a result is created, there is currently no way to add attachments to it. Isn't it even more important to have attachments during the actual project than during the budgeting phase?

This has been so far requested by about 50% of our customers.

**Describe the solution you'd like**

I'd like to have attachments and folders with accountability results

**Describe alternatives you've considered**

Using external file management services and linking to those documents. But this is inconvenient as we have the attachments feature already in Decidim.

**Additional context**

Pull request: #6851

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

No.

**Funded by**

Helsinki (external module) and Turku (core integration).