Discourse revisited

Hi all,

In #46 the community discussed starting a Dask discourse page. There were some initial experiments in that direction, but then the discussion seems to have stalled, and the page lapsed. I'd like to revisit the idea.

There are many valuable discussion topics that are (in my opinion) not particularly well captured by the current set of communication tools (GitHub issues, Slack, Gitter, Stack Overflow). These include questions about usage, configuration, deployment, integration with other tools, and general advice for how to approach Dask-based workflows. We [currently](https://docs.dask.org/en/latest/support.html#discussion) point users towards Stack Overflow for usage questions and Gitter/Slack for chatting (though the latter are somewhat discouraged since they have a poor search story).

At this date, Discourse forums have been adopted by (at least) [Jupyter](https://discourse.jupyter.org/), [Pangeo](https://discourse.pangeo.io), [Bokeh](https://discourse.bokeh.org), [PyMC](https://discourse.pymc.io/), and [Plotly](https://community.plotly.com/), most of which with decent-to-good community engagement. They seem to be a well-understood and liked model for community discussion, and have at least a few major advantages over the current community slack:
* One can view and search for discussions without having an account/slack invite (I suspect this is a major impediment to attracting more community discussion in the slack).
* The discussions are searchable by users and indexed by search engines (I regularly see the Jupyter discourse pop up in my google searches)
* It's easier to [categorize and tag](https://blog.discourse.org/2017/10/its-time-we-talked-about-tags/) topics.

### Alternatives

**GitHub issues**: This repository aside, they are intended for bug reports and feature requests, usage questions are discouraged.

**Stack overflow**: can be an excellent Q&A website, but has a very strict definition of what constitutes an on-topic question. A Dask-specific forum allows for a wider range of conversation.

**Gitter**: mostly discouraged due to it's poor searchability and ephemeral nature.

**Slack**: better used and understood than Gitter, but many of the same drawbacks.

### Specific proposal

I'd like to resurrect the [dask.discourse.group](https://dask.discourse.group) Discourse page and try out some layouts, categories, tags, and welcome messages. If people like it, we can "bless" the page and start directing traffic there. Of course, it would only work if we are able to have knowledgeable people monitoring it and maintaining a welcoming and civil atmosphere. I would hope to draw from the experiences of some of the above communities.

### Objections
* *I already have too many communications channels*: fair point, though I think there is a real gap in the Dask community channels today. Worth a shot?
* *Who would maintain and moderate this board?*: hopefully this would be a group effort, but I would be happy to start off doing this. This would largely be under the auspices of Coiled (my employer), but I would hope to be able to attract other institutional and individual contributors.

Thoughts?