First working version of PPLNS payouts

* Based PPLNS on Prop Payout script
* Using defaults from prop payout, no class adjustments
* Added more methods required for PPLNS
* Added block methods for dynamic payout calculations
* Added PPLNS Sidebar that also displays the PPLNS Target
 * Shares beyond this target will not be included in payouts
 * Shares missing to this target will be added from archives
* Enabled archiving by default for PPLNS
* Added configuration options for PPLNS
 * Documented the usage for PPLNS, defaults are sane
* Added pplns_payout to run-crons

Addresses #143 and if accepted will fix it
Abort payout process if missing share_id

As discussed in #392 we should abort any payouts for rounds until the
issue is fixed.
Fixing est. for round in PPLNS sidebar

Fixes round estimates for PPLNS. These are pretty much based on props
but should still get close to the actual payouts.

Addresses new issue in #143