pi: Allow for billing status error correction.

[amass01]

Background:

Let's say an admin makes a mistake and sets the billing status to
completed when they actually meant to set it to closed. There should be
a way for them to correct this mistake.

We would like to enable error correction by adding a plugin setting
which limits the number of the allowed billing status changes. It
defaults to 1 and can be increased by a sysadmin in rare cases where a
billing status error correction is needed.

Implementation:

• Adds allowedbillingstatuschanges pi plugin setting which defaults
  to 1 and can be increased if a billing status error correction is
  needed.

• Adds in billing status validation that allows for the following state
  transitions:

  • active can only be set to closed or completed.
  • closed and completed can be set to any other billing status
    when the plugin setting allows for it.

• Adds active status to the pictl proposalsetbillingstatus command.

---

Closes #1525.

[amass01]

@lukebp updated 💯

[lukebp]

I'm getting a pi summary 500 if I've updated the billing status more than once.

2021-10-07 10:50:41.375 [ERR] POLI: 127.0.0.1:34688 POST /v2/pluginreads HTTP/1.1 Internal error 1633621841: PluginRead pi summary : couldn't determine proposal status of an approved propsoal: billingStatus: 1

Fix the propsoal typo in the error message too.

[amass01]

@lukebp

Fixed by introducing:
func proposalBillingStatus(vs ticketvote.VoteStatusT, bscs []pi.BillingStatusChange) pi.BillingStatusT { }
Which determines the billing status of a proposal.

[lukebp]

tACK

Just need to fix the formatting nit.

[lukebp]

tACK