profile: drop gravatar support

gravatar is owned by an external entity (Automattic) and we have an
unnecessary dep to this service. users can just upload a profile pic

src/test/common.js

@@ -102,7 +102,7 @@ const admin = {
     groupIds: [],
     role: 'owner',
     source: '',
-    avatar: constants.AVATAR_GRAVATAR,
+    avatar: null,
     active: true,
 };
 
@@ -119,7 +119,7 @@ const user = {
     groupIds: [],
     displayName: 'Normal User',
     source: '',
-    avatar: constants.AVATAR_NONE,
+    avatar: null,
     active: true,
 };