slack: add CP channel prefix

[sprnza]

SUMMARY

1. For channels wich ID starts with CP # symbol is being prepended
2. docs improved for cases when message_id is supplied

SUMMARY

1. CP string was added to the list which describes channel_id prefixed for which # symbol will be added in front of channel_id
2. Added notes regarding channel_id has to be literal Channel ID returned by previous task run.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

slack

ADDITIONAL INFORMATION

1. When build_payload_for_slack is called there is an if which prepends # if channel_id starts with a certain symbols. I've stumbled upon a channel which ID starts with CP but current implementation doesn't have this string in the list so I got #CPxxxxxx added to payload.
2. It fails when message_id is supplied and channel_id is either "mychannel_nameor#mychannel_nameit only works. Whenchannel_idis in form ofC0xxxxxxx` which is returned by previous task run.

[ansibullbot]

cc @ramondelafuente
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[sprnza]

Unsolved root cause of issues is field channel doesn't accept channel name when message_id is provided.

[felixfontein]

Thanks for your contribution. Could you please add a changelog fragment? Also please fill out the PR form the next time instead of removing all the fields you find irrelevant. Thanks.

[sprnza]

@felixfontein Done! Sorry about not following the rules. I've also added a docs change for issue I've also stumbled upon and it wasn't described in the docs.