Fix #219: either use the bot token or the webhook URL

[filmaj]

... but do not use both.

I believe this mainly affects this repo's integration tests. I was noticing that the bot-token based integration test (integration_test_botToken) was posting to a test channel of mine twice... turns out it was because I had both the bot token and a webhook URL defined as environment variables when testing locally.

Fixes #219.

I think this may need some work, though. The intention in this fix is to ensure each time this action is consumed as a step, we don't do multiple things (post a message via the postMessage API and also HTTP POST to a webhook URL). However, the API in this Action for either posting message vs. HTTP POSTing to a webhook is very similar, and so it is hard to untangle the two if you want to use both in a single workflow action file. Maybe it is worth deferring this work to a v2 of this Action with a new API that more cleanly delineates these different behaviours?

[seratch]

LGTM; thanks for fixing this!

[stevengill]

LGTM

[zimeg]

LGTM! The TODO around returning message information for only one of many messages is a nice catch. I wonder if there's some slick way to include information of multiple messages 🤔

Maybe it is worth deferring this work to a v2 of this Action with a new API that more cleanly delineates these different behaviours?

I agree that a new API for this can be more clear, but don't think that should stop this PR!

Also wondering if a note in the README should be added to say something about the SLACK_BOT_TOKEN taking priority over any SLACK_WEBHOOK_URL? Unsure if the current behavior is relied upon, but this might save future confusions with this!