[Feature] Add a possibility to filter-out cards with a label

[s-and-witch]

Which part of the project does this feature apply to?

Frontend

What would you like?

I'd like to be able to filter out a label, i.e., show only the cards that do not have the label I selected. There should be three possible states for a label in the "filter by label" view:

1. default (does not affect filtering)
2. positive (show me cards with this label)
3. negative (show me cards without this label)

I'm not sure how this feature should be implemented UX-wise, because it's convenient to have on/off filtering with one click, and adding a third state would break the workflow for some users. There is plenty of room for discussion.

Why is this needed?

This may be useful when you use labels to mark subsystems and want to see all the subsystems except one specific subsystem that you're not interested in. For example, you have a project with frontend issues, backend issues, and unspecified issues, and you want to see only backend and unspecified issues.

Another example is when you want to introduce a new tag to all existing features and display only the features that haven't been marked yet.

Other information

I may implement this feature code-wise if you point to the code parts where filtering happens; however, I'm not a frontend developer, and I'm hardly capable of building a good UI.

[meltyshev]

Hey! That's a nice thing to have. I agree that the challenge is how to present it in the UI, I'll try to think about that, or maybe someone else will have ideas. Also, this filter should work on the server side as well, since we use server-side filtering for the archive/trash lists.