Change default_priority default value to None

[ryneeverett]

Currently, default priority is a common service configuration option which defaults to M. It is generally agreed that a better default would be None -- that is to assign no priority to tasks by default. 1 2 3

The challenge is in doing this gracefully. To quote myself:

I agree that this would be a better default but I'm not sure there's a graceful way to make the change at this point. This would be a drastic disruption of the priority economy in existing databases in which users may have already adjusted their coefficients in line with bugwarrior's default...

However, I now think this is bothersome enough to be worth pursuing, even if it does take a long time and several release cycles. I propose the following:

• Add a top level default_priority setting with a default value of M which is the default value of the per-service default_priority options.
• Make this setting required. (In a future major release of bugwarrior. 3.0?)
• Make this setting optional with a default value of None. (In a subsequent major release of bugwarrior. 4.0?)