Create subclasses of `Issue` class, for the various phases

Currently, the `Issue` class contains properties and methods for issues used in all 3 phases (technically 4 phases, including the moderation phase). The properties / methods used for the different phases can be quite unrelated.

This can make the class difficult to understand.
In future, the class might become even more complex if we add more unique functionality to each phase.

Let's consider the possibility of creating subclasses of `Issue`, and moving the phase-specific properties & methods into these subclasses.
There will be some common code left in the `Issue` class (example: initialising the `Issue`'s state based on its labels).

Start by locating the Issue class and cataloguing its properties and methods by the three main phases and moderation. Review how Issue objects are created and used, then determine the shared behavior that should remain in Issue and the phase-specific behavior that would move to subclasses. Done means the design is agreed and the phase-specific responsibilities are separated without breaking existing usage.

Written by the indexing model from the issue text.