Allow admins to "internally" categorize proposals into different scopes

Is your feature request related to a problem? Please describe.
Currently Decidim allows admins to change the category or scope of a participant submitted proposal but it limits the admins to the same selections that are available for admins.

When the amount of proposals increase, it becomes hard for admins to manage those internally and they would like different types of categorization options that are not visible for the participants but would be visible for admins and enable admins to search with these categories/scopes and include this information also in the exported data.

Describe the solution you'd like
It came out as an actual customer request that they would like to use the "scopes" feature that we have today for proposals but so that it would be only visible for admins.

This would be different than the scope that the participant selects when they submit the proposal. These controls would be only visible for admins within the listing view of proposal where they can already change the category or scope of the proposal. We could add more options there, so that the admins could use "internal scopes" for categorizing the proposals even further.

One main point that came up in the conversation was that typically these are not something that the participants would even be able to define, they don't know e.g. to which "department" a certain proposal belongs to. Even the admins may not know it directly but when they are evaluating the proposals with different departments, this information is usually defined at that stage.

After the "department" (or some other categorization) has been defined for the proposals, the admins would like an easy way to list proposals belonging only to that department and also export the proposals of certain department.

We would see that the "scopes" feature could be used for this need, as this information does not typically change process to process. This is relatively static information that only changes in case the city faces an organizational restructuring. Therefore, the scopes would be more suitable for this than categories.

Describe alternatives you've considered
It was actually the customer's own idea that they can "hack" their way with the current scopes feature so that they disable scopes during the proposal collection phase and once proposal submission is over, they will enable scopes which allows them to do exactly what they need.

But this comes with a couple of problems:

1. The scopes feature becomes unusable during the proposal collection period, e.g. if they would like to also categorize the proposals for certain areas (South, East, West, etc.) through the scopes as well as to certain themes (parks & nature, sports & outdoors, built environment, etc.) through the categories
2. The admins cannot "scope" the proposals during the proposal collection period if they already want to start evaluating the proposals after they receive them

Additional context
There are a couple of previous proposals that have been submitted that are after something very similar but not quite (I was looking if someone had already suggested something similar):

1. Tag system for administrators by Simon
2. Allow mapping multiple scopes to resources by me and Anthony

Particularly the second proposal would allow implementing something like this more easily if we did not limit the scope selection to only one. But it would not completely solve this problems as these admin scopes should not be visible for the participants when submitting the proposals.

Does this issue could impact on users private data?
No

Funded by
N/A