StateManager conditional states need labels and priority over the main state

[jace]

In Funnel, we have a conditional state named SCHEDULED which builds on the CONFIRMED state and confirms a linked session in the schedule.

SCHEDULED takes precedence over CONFIRMED in the UI. A proposal that is scheduled should be listed as such and not as merely confirmed. This requires three changes:

1. Conditional states need labels, similar to the labels inherited from the LabeledEnum (including NameTuple support). add_conditional_state needs to accept a label parameter.

2. proposal.state.label (in Funnel) should return the conditional state's label instead of the main state's. To achieve this, conditional states should have a priority field, should be pre-sorted by priority, and should be tested in order of priority to find the first match. Main states have a priority of 0 and conditional states should get max+1 unless manually specified. This will allow more than one conditional state to exist on the same main state.

3. Sometimes we'll want a UI construct like this: {% if proposal.state.SCHEDULED %}{{ proposal.state.SCHEDULED.label %}{% endif %}. This can't work because state.NAME is a boolean. What if it is a ManagedStateWrapper instead, which in turn implements __nonzero__/__bool__? The if condition will continue to work then.

[jace]

We don't need a priority field if we just use definition order and make later definitions higher priority (so conditional states defined after the main state also take higher priority).