Only accept webhooks with a matching livemode setting

[polski-g]

What's this PR do?

Adds an optional configuration option to only accept events if the event "livemode" parameter is set to a value we're interested in.

Any background context you want to provide?

Per Stripe (https://stripe.com/docs/connect/webhooks):

For Connect webhooks, it’s important to note that while only test webhooks will be sent to your development webhook URLs, both live and test webhooks will be sent to your production webhook URLs. This is due to the fact that you can perform both live and test transactions under a production application. For this reason, we recommend you check the livemode value when receiving an event webhook to know what action, if any, should be taken.

So we need to be really sure that we want to accept a webhook event for the given environment.

What ticket or issue # does this fix?

None that I know of.

Definition of Done (check if considered and/or addressed):

Code is backward/forward compatible; default setting is None, which means do not check "livemode" parameter at all.

No new dependencies.

Documentation not updated in this PR.

I don't know how to write unit tests or run the test suite, so not done.

[codecov[bot]]

Codecov Report

Merging #547 into master will decrease coverage by 0.16%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #547      +/-   ##
==========================================
- Coverage    99.4%   99.23%   -0.17%     
==========================================
  Files          33       33              
  Lines        1838     1841       +3     
  Branches      168      170       +2     
==========================================
  Hits         1827     1827              
- Misses          5        7       +2     
- Partials        6        7       +1

Flag	Coverage Δ
#py27dj110	98.91% <0%> (-0.17%)	:arrow_down:
#py27dj111	98.91% <0%> (-0.17%)	:arrow_down:
#py27dj18	99.18% <0%> (-0.17%)	:arrow_down:
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#py35dj111	98.91% <0%> (-0.17%)	:arrow_down:
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Impacted Files	Coverage Δ
pinax/stripe/webhooks.py	98.17% <0%> (-0.91%)	:arrow_down:

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[paltman]

Good idea, but I think we should probably just discard in the view before even creating the Event