Outfactor component logic in ManageLabelsModal

[XanderVertegaal]

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 #64 (comment)

[jgonggrijp]

This is not only about the tests. I meant to say that the functionality doesn't really seem to belong in the component.