Add Slack Events backend

[duhow]

This is a copy of Slack RTM adapted to work with Slack Events v2.
It creates a Webserver webhook manually to listen to Events sent by Slack.
It is using latest library slackclient>=2.
There are few changes, such as username become user full name - Slack API deprecates name field and intends to use display_name instead, paging while loading all channels, caching some functions to avoid querying information at all times...
Not complete, but enough to interact with messages! :)

[nzlosh]

I think this duplicates the work here #1451

[duhow]

Oh, didn't know there was another PR 😮
This backend was waiting for too long and I needed to write it recently for replacing my old bot.