Caseflow: create new scope in `Task` that finds actionable tasks that filters out `on_hold` tasks

[youngfreezyVA]

create a scope in Task that finds actionable tasks - this would entail renaming active to open and use active to mean actionable

• update all usages/instances of tasks.active.where.not(status: :on_hold) to use this new scope

• update most_recently_assigned_to_label in app/models/appeal.rb to use this new scope

• fix the 30+ failing tests as a result of this change

[lowellrex]

Could we include renaming inactive to closed as well? So we end up with the following taxonomy:

Where we currently have:

[youngfreezyVA]

sounds good! that makes sense. i'll also follow the bullet points above and look to replace usages of the old taxomony of closed/inactive