Replace Matrix and Slack links to Swung's Mattermost

[santisoler]

Replace links to the Matrix chat room and the Slack channel in Swung for a new
Mattermost Team in SWUNG's instance.

[santisoler]

We should create a Fatiando team in swung's mattermost before merging this PR.

[leouieda]

Seems like most others are creating a channel in the main SwUng team. Should we do the same or go with creating another team. I'm afraid of people getting lost in the difference between servers and teams.

[santisoler]

I haven't made my mind on choosing either a team or a channel. You're right that people might get lost in the confusion, so we might want to create a channel (just like what we have now in Slack).

@MGomezN @LL-Geo @aguspesce what do you think?

[MGomezN]

Yes!
I vote to keep it similar to the current state. :)

[LL-Geo]

I like the channel option. So people can easily move around within the SwUng team 😁

[santisoler]

Cool. I've just created the fatiando channel in Mattermost and added a few links to the header. I'll update this PR then.