Disable Slack Wysiwyg per Ross Jacobs

UPDATE (2019-12-18): The script this extension is based on no longer works (as of Slack version hash 0756f06a6f0457de4e355149a4321a9abb8befa5). For more information (or to help fix this), read the issue: https://github.com/kfahy/slack-disable-wysiwyg-bookmarklet/issues/27.

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Disables Slack Wysiwyg 'Feature'.

## Background

Slack used to have the ability to use markdown in their text. They changed this permanently for all users without the ability to go back. Per Arthur O’Dwyer on his blog entry that was posted on [Hacker News](https://quuxplusone.github.io/blog/2019/11/20/slack-rich-text-box/) on 2019-11-20):

> Slack has just recently rolled out a “WYSIWYG text input” widget to its Web browser interface. (Apparently, the phased rollout started at the beginning of November 2019, but it’s just now starting to hit the workspaces that I participate in.) The user experience of using this new input method is really, really, really bad.

Inspired by the 2 Hacker News articles today: https://news.ycombinator.com/item?id=21589647, https://github.com/kfahy/slack-disable-wysiwyg-bookmarklet

## Source & Credit

Github: https://github.com/pocc/no-wysiwyg
Based on Kevin Fahy's script at https://github.com/kfahy/slack-disable-wysiwyg-bookmarklet.