Slack backend be able to react on `message_deleted` event

[kissmikijr]

In order to let us help you better, please fill out the following fields as best you can:

I am...

• Reporting a bug
• Suggesting a new feature
• Requesting help with running my bot
• Requesting help writing plugins
• Here about something else

I am running...

• Errbot version:6.1.8
• OS version: macOS Catalina 10.15.6
• Python version: 3.7.9
• Using a virtual environment: yes

Issue description

Please describe your bug/feature/problem here.
I'd like to make actions when a certain message gets deleted. I think it would be nice if it could call a callback like the callback_reaction so devs could decide how to act on this event.

Additional info

I'm happy to work on it if you could help me out.

[sijis]

This is an interesting feature. I'm not against having this callback available for backends.

You could look at this PR as inspiration on what may be necessary to add this functionality

errbotio/errbot#1500