Link to matrix space

Since we have it set up and some people are reluctant to join discords, I think we should share both the matrix and discord links in the header. But we'd need to do it in a way where it is obvious that they are bridged.

Unless we have better ideas, I'm thinking that we can change the Discord community button to

which becomes a drop-down menu with

- [Discord](..)
- [Matrix](..)
- Discord and matrix are bridged, pick your faviourite

And on mobile, I guess those items could be indented.

@sornas would you mind taking a look at this at some point. I don't feel like webdeving right now :D