Better handling of syncing deleted customers

[sprocket-9]

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)?
• Have you written tests to prove this change works (if applicable)?

[sprocket-9]

@tasn used cherry pick on your commits and resubmitted this PR - does this look OK to you?

[tasn]

This looks great. Thank you.

Also, extra thank you for adding the tests to finally get this PR merged in!

Edit: forgot to mention, I'll now abandon my PR in favour of this one.

[sprocket-9]

@tasn great! Learned a lot of new git skills doing this too 👍

[tasn]

I was going to explain to you how to do it, but since you said you were new to git, I thought it may be too complex.

Glad you managed to do it on your own and learn new tricks on the way. Git is a powerful tool, you can do very cool things with it.

[paltman]

Just a small thing, but can you change to double-quotes to keep consistent with project style? Thanks!

[sprocket-9]

@paltman np, done!

[sprocket-9]

@paltman added a commit to use double quotes so this PR is good to go.

[tasn]

Thanks everyone. Happy it's finally in!