Pass payload into the context function for subscriptions

[clayne11]

The payload can be used for things like auth. You can pass a token on
the payload of the message and use it to fetch the user.

Fixes #1505.

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[evans]

@clayne11 Thank you so much! We'll get this out in a 2.1 release.