Prepare changelog for v1.5.12
Prepare changelog for v1.5.12

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