Add Net Promoter Score feedback support

[egamma]

CC @seanmcbreen

fwlink to the survey: http://go.microsoft.com/fwlink/?LinkId=823149

[joaomoreno]

@egamma @seanmcbreen Can I get a review over the following description?

We should propose the user to answer the NPS score after a fixed amount of sessions. The proposal should occur with a certain degree of probability. If the survey happens to not be shown, it will not be shown again to this user. The user can opt to either get asked again later or not get asked again.

• A session is an execution of VSCode, capping at the maximum of one per day.
• The survey should be proposed at the 9th session.
• The probability of being proposed is 15%.
• If the user requests to do the survey later, we should propose it at the 12th, 15th session, etc.
• Any of following outcomes will result in never asking the user again (until we release a new version that overwrites this):
  • User clicks the Don't ask again button
  • User clicks the Yes, let's answer the survey button
  • User is part of the 85% of users that didn't get selected for the survey proposal

[egamma]

@joaomoreno this captures well what we discussed, but I defer to @seanmcbreen since he did the spec.

[joaomoreno]

Pushed 5bcde31 containing the logic described above. nps.contribution.ts is the interesting part.

[joaomoreno]

I've also set the npsSurveyUrl key in the product.json of both stable and insiders, in the distro repo. Should it be the same URL, @seanmcbreen?

I'll assign this to someone, needs a code review.

[joaomoreno]

@jrieken Can you review? Let me know if you'd like me to run you through the code, might be easier.