Allow mapping multiple scopes to resources

Is your feature request related to a problem? Please describe.
Currently Decidim only allows mapping a single scope to any given resource.

In many cases it would be useful to allow mapping multiple scopes to different resources.

A concrete example of this can be seen on the MeinQuartier.Zuerich instance on this page:
https://meinquartier.zuerich/assemblies

Some of the assemblies are related to multiple areas of the city but Decidim only allows mapping them to one area. For example, the following assembly should be mapped to two city areas (Oerlikon + Seebach):
https://meinquartier.zuerich/assemblies/drehscheibe-oerlikon-seebach

Another problem regarding this is that the Areas feature is officially deprecated as per the following discussion:
https://github.com/decidim/documentation/commit/50f0f25ddbb0e013df6f9c765d2e1da8af0cef29

Therefore, we suggest implementing this feature through scopes instead.

Describe the solution you'd like
We would like to allow mapping different "scopable" resources in Decidim to multiple scopes, not only one.

This would require technical changes in how we store the scopes mapping as currently the different resources have direct one(scope)-to-many(resources) relations for the scopes. We would like to create a similar mapping for the scopes what we today have with categories ("decidim_categorizations").

This way, in the particular use case we could utilize the scopes concept to allow mapping the assemblies to multiple areas.

Even though the need here is only to map assemblies to multiple scopes, this change would consist of allowing any scopable resource to be mapped to multiple scopes:

• Participatory spaces (processes, assemblies, etc.)
• Components
• Scopable resources (proposals, meetings, etc.)

Describe alternatives you've considered
Implementing the same change for areas but it does not look very sustainable change in case areas are deprecated as per this discussion:
https://github.com/decidim/documentation/commit/50f0f25ddbb0e013df6f9c765d2e1da8af0cef29

Additional context
This feature proposal only covers the technical ability for mapping resources to multiple scopes.

Deprecating the areas concept is outside of the scope of this change. But the idea would be to implement this in a way that would allow deprecating the areas concept later down the line.

Does this issue could impact on users private data?
No.

Funded by
City of Zurich