Matrix/Element.io instead of Gitter + Telegram for community communication

Latest update: we're test-driving Matrix using Element.io. There's a community with two rooms setup here: https://matrix.to/#/+decidim:matrix.org.

Waiting on response from the association committee to move forward with more rooms and migrate in practice.

I've updated the title of the proposal to reflect where it's currently at.

-------------------------

(This not a feature proposal - posting it here as it seems to be the best option at this point. It's definitely meta, at least)

So, I propose migrating community communications now done on Telegram and Gitter over to a new Decidim Slack. 

I believe this would greatly improve support to users and developers, better enable water cooler conversations and brainstorming, where a lot of good ideas come from, and increase overall governance transparency. It'd strengthen community integration.

Though I like the intention of Gitter and the fact it's now on Matrix, it's still very clunky and it'lll be a long while until it's on par with Slack. The Telegram groups are hard to find (a lot of people posting on Gitter, Meta and Github don't know they exist) and don't allow for separating topics into channels that can be easily browsed.

With Slack we can have channels like 'new-to-decidim' or 'getting-started', 'core', 'deployment', as well as 'product' and 'general' (for announcements, participatory ecosystem news, etc). Also, specific topics like 'e-voting' or 'participatory-budgeting', and even locale or region-specific channels. 

We can have automated notification channels tracking Github activity (commits, issues, PRs, etc), and even Metadecidim proposals. Integration for the latter would need to be developed, but it could come handy to Decidim instance communities as well.

It wouldn't need to be persistent (the conversations are meant to be ephemeral and can be thrown away), so it doesn't need to be paid.

For some context, this idea is something I discussed privately with @andres a couple of months ago, and briefly mentioned to @carol as well. We agreed it was better to wait a little bit before starting this discussion until after the holidays, with the association in place and with the election for the coordination committee over.

@carol also said there might be alternative proposals floating around - so perhaps this can start a conversation on the different options and relative benefits. 

Rocket.Chat might be one, or even Matrix itself. I've never used them myself and so can't really vouch for them. I know OSP uses Rocket on their daily work, perhaps they might chime in with their experience with it here. I suspect Matrix is hard to use, but I could be wrong. Another option is Discourse, but then we wouldn't have the quick, real-time chatter that Slack offers.