follow-up to privacy modes: open invites

[cryptix]

initially part of #96 but while talking with @cblgh about #100 I realized that the invites implementation needs to be overhauled to conform with ssbc/rooms2#35 first anyhow.

I suspect we don't strictly need invite creation (which is currently tied to members here) but basically have open consumption that makes people internal users no matter what the inviteCode.

So in open mode we could have a Create Invite button somewhere that just returns a link with a random code.

[cryptix]

Related: #13