🎩 What? Why?

Fixes exception on user notifications page. the issue is on generating of proposal_url for a notification when proposal related to anything else except the participatory process.

🐞 The bug

The notification was introduced here and the related tests passes because in tests proposal is by default created for participatory_process.

So it works well with processes, and decidim_admin_participatory_process_proposals engine can properly generate the url for the proposal added to the participatory_process.

The thing is that proposals can be added not only to the processes, but to a few other things, like assembly, initiative, consultation, etc:
https://github.com/decidim/decidim/search?q=%22include+Decidim%3A%3AParticipable%22
(check for include Decidim::Participable)

In those cases the engine would be a different one (accordingly to the component, decidim_admin_assembly_proposals for assemblies, for example). This results to the error when rendering the ProposalNoteCreatedEvent notification:

     ActionController::UrlGenerationError:
       No route matches {:component_id=>113}, missing required keys: [:participatory_process_slug]

Participatory processes engine can't generate urls for the assemblies, and vise versa.

🔧 The fix

The fix is to use the mounted engine that is different depending on the proposal component, it picked from the Participable concern:

Testing

1. Create an assembly
2. Add proposals component to the assembly
3. Create proposal for that component
4. Add a proposal note to trigger the event
5. Check the notification page as any user who follows that assembly (as admin user should be enough, it follows by default)