Allow creation of sublabels

Currently labels are in a plain level, which works well UI wise when the amount of labels doesn't exceed around 10 labels. However, there are few cases where the amount of labels explode into bigger numbers and assigning them becomes difficult given the cluttered UI. These labels are usually more specific and can be grouped, and therefore sublabels would help.

The two cases that we found are:

• Translations: A label is created for each language, which helps translators to subscribe to the language they are interested in.
• Components: A label for each part of the project, which helps on keeping bugs organized and being able to subscribe to specific parts of the project.

An example how the UI could look like:

When clicked on Component:

And you can navigate back clicking the "Component" title

Then translations would looke like:

And same process to return back.