Improve category navigation in proposals

[josepjaume]

🎩 User Story

As stated by @xabier in https://github.com/AjuntamentdeBarcelona/decidim/issues/1089, navigating categories should rethought in a way that:

1. Gives a clear image of the structure at first glance
2. Allows filtering by multiple categories/subcategories.

📋 Related documentation

None

🎯 Acceptance criteria

Filtering by categories is more ergonomic & powerful.

📌 Related issues

https://github.com/AjuntamentdeBarcelona/decidim/issues/1089

[xabier]

@josepjaume can we quickly make a simple, temporary, workaround by including something like "Filter/Navigate by categories" as a title or visible content (when non-selected) of the dropdown menu? right now it is blank so users don't even know what might be inside the dropdown.

[josepjaume]

It looks like we're missing a prompt for the select dropdown - I've added that as a bug: https://github.com/AjuntamentdeBarcelona/decidim/issues/1165

We'll see how can we improve this in the future.

[xabier]

The future is now 😄 this feature is relatively simple and high impact for usability