Add support for using "libravatar" as the GravatarSource (#3969)
Add support for using "libravatar" as the GravatarSource (#3969)

Just to make it easier for administrator to configure libre avatar,
as it is done for "duoshuo" and "gravatar"
dancewing/gogs,cgars/gogs,xaionaro/gogs,dlob/gogs,gogits/gogs,xaionaro/gogs,liliexuan/gogs,cgars/gogs,mukaiu/gogs,Spice-King/gogs,dlob/gogs,ForxaCoLab/forxa,chriswatt/gogs,linquize/gogs,chriswatt/gogs,mukaiu/gogs,pikacloud/pikacode,wangjild/gogs,psaavedra/gogs,dlob/gogs,wangjild/gogs,chriswatt/gogs,dancewing/gogs,linquize/gogs,gogits/gogs,Spice-King/gogs,wangjild/gogs,ForxaCoLab/forxa,psaavedra/gogs,xaionaro/gogs,pikacloud/pikacode,ForxaCoLab/forxa,Spice-King/gogs,mukaiu/gogs,cgars/gogs,dancewing/gogs,linquize/gogs,liliexuan/gogs,gogits/gogs,pikacloud/pikacode,psaavedra/gogs,dancewing/gogs,liliexuan/gogs