Prepare changelog for v0.15.3
Prepare changelog for v0.15.3

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