Prepare changelog for v0.12.0-rc.1
Prepare changelog for v0.12.0-rc.1

martindale/vector,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,martindale/vector,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web