make the api (/{login,*}) only use the `:domain.tld` host endpoint (not matrix.org, or any other server)

• https://gitlab.com/sctlib/matrix-room-element/-/blob/main/js/api.js#L198

• https://gitlab.com/sctlib/matrix-room-element/-/blob/main/js/api.js#L98

• see the this.defaultBaseApi (hostname?)

• in @user:domain.tld, we should force the use of domain.tld as the host, to authenticate a user

that way their credential are only shared to their homeserver, and not matrix.org (like currently)

• Also, should be the same for sign-up (not implemented) and sign-out

• Also, add the possibility for matrix-login and matrix-logout (sign-up too), to enforce/find only one homeserve, like in libli