Gitter Chat URL redirects to gitter.im homeserver

[golanv]

The current URL in core-spacemacs-buffer.el implies a gitter.im account is required to access the chat (which is not the case since Gitter moved to Matrix). A more standardized way to handle Matrix links these days seems to be to use https://matrix.to (or a comparable service/instance). Perhaps this link should be changed to "https://matrix.to/#/#syl20bnr_spacemacs:gitter.im"?

Additionally, should the tag be changed from "Gitter Chat" to something more generic (e.g., "Community Chat", or something)?