Better handling of syncing deleted customers

[sprocket-9]

Credit to @tasn for work done in PR #325 - this PR adds tests

Fixes #268

What's this PR do?

Modifies deleted customer webhook to purge customer locally and not attempt to delete customer again on Stripe.

Modifies customer syncing actions and management command to handle situation where customer is deleted remotely but not locally.

Any background context you want to provide?

What ticket or issue # does this fix?

Closes #268

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

• Are all backwards incompatible changes documented in this PR?
• Have all new dependencies been documented in this PR?
• Has the appropriate documentation been updated (if applicable)?
• [x ] Have you written tests to prove this change works (if applicable)?

[sprocket-9]

hey @tasn,

New to git and wasn't sure how to proceed so asked the project owners in slack who said a resubmit and acknowledge your work in the PR would be OK.

[tasn]

Thanks for your reply, but it's not for them to decide, but rather the author (in this case me).

If you give me access to your forked repo, I can fix the PR for you.
Alternatively, the easiest way would be to amend your commit to remove my fixes, and have this PR only include the tests, and the maintainers would have to merge mine first.

[tasn]

I can see you submitted a new PR (#358). Please close this one.

Just to let you know, you can update a PR by (force) pushing to the PR branch, no need to start a new one.