Outfactor component logic in ManageLabelsModal

~Not all tests that are currently written as component tests (e.g. those for `getAttachedByText` in the `ManageLabelsModalComponent`) should be component tests.~

Parts of the logic that are now in the ManageLabelsModal are not strictly tied to that component. They should be outfactored.

> These are tests for a component, which is a view and a controller in the MVC triangle, but I think the tested behavior belongs in the model layer. Indeed, it looks like something that could fairly easily exist indepently of the component.

_Originally posted by @jgonggrijp in https://github.com/CentreForDigitalHumanities/langpro-annotator/pull/64#discussion_r2683922944_