swarm: transform non swarm to swarm conversations

+ Remove readOnly from conversation's as it was never used
and not necessary in multi swarm as you can re-add members anyway
so it's never read only.
+ Makes non swarm conversations just readable.
Users can save whatever they want and replace old non migrated
conversations to new swarm.

Change-Id: I8aa2521bc524b6159d7ba0b613834438e293e09e