Slack channel fixes

[Maxinho96]

Proposed changes:

• Fixed response when slack sends retries (None was giving error)
• Running on_new_message as an asyncio background task instead of a blocking await (so that we immediately return 200 to Slack and we don't reach the 3 seconds timeout if the action execution is slow)

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[ancalita]

@Maxinho96 Thank you for your contribution 🙌🏻
Looks great 💯 Before I approve this PR, I would like you to:

• add a unit test that fails when you revert your changes and passes with your fixes (the unit test should be placed under tests/core/channels/test_slack.py)
• add a changelog entry in the changelog directory, you can use the PR id as changelog entry ID, please use type bugfix.

Also please note that since your PR is targeting main, this change won't be released until the next minor and that is tentatively scheduled towards the end of Q3. Alternatively, you can recreate your PR to target 3.2.x and we can release a micro much earlier.

[Maxinho96]

@ancalita done :)

[ancalita]

@Maxinho96 To double-check did you also rebase your branch locally and then forced push? It seems the PR now contains a lot of extra commits 😬 This will have to be fixed before I can review again.

[Maxinho96]

@ancalita Sorry, I have fixed with a rebase :)

[ancalita]

Thanks for the speedy implementations, looks great 💯
I just have two more suggestions for expanding a bit on the issue and solution in the changelog entry & for refactoring a bit the tests. Let me know how you get on.

[ancalita]

🎉