add screenshot support for Slack Handler

[ramaarf]

I've add screenshot support for Slack Handler, the user need to provide api token to access Slack Api files.upload

[ThexXTURBOXx]

Thank you very much for your PR! Have you tested this somehow?
Also, there might be the possibility for a race condition: If two users crash at (almost) the same time, the error message of user A will appear and it will start uploading the screenshot, but in the mean time the error message of user B will appear and he will also start uploading. Only after that the screenshots will follow - also in some random order which is dependent on the internet connection of user A and B.
The worst possible case would be the following order:

Error message of user A
Error message of user B
Screenshot of user B
Screenshot of user A

Is it somehow possible to send a message with a screenshot in a single API call such that the screenshot is linked to the given error message?

[ThexXTURBOXx]

Here it looks like they are additionally uploading an image with the message. Maybe that would be a better approach?

[ramaarf]

Afaik, to upload file on slack we need to call it separately via https://slack.com/api/files.upload, the attachment only work with image url not uploading with the message. So I've made an update by changing the flow to upload the screenshot first to the channel, then linked the url to the error message as attachment.

[ThexXTURBOXx]

I have tried a few things and could not get it to work any better than that. So, I will publish it soon. Thanks for the contribution! :)