use correct partial indexes on project token fields

services/web/app/coffee/models/Project.coffee

@@ -33,8 +33,20 @@ ProjectSchema = new Schema
 	imageName         : { type: String }
 	track_changes     : { type: Object }
 	tokens            :
-		readOnly        : { type: String, index: {unique: true} }
-		readAndWrite    : { type: String, index: {unique: true} }
+		readOnly        : {
+			type: String,
+			index: {
+				unique: true,
+				partialFilterExpression: {'tokens.readOnly': {$exists: true}}
+			}
+		}
+		readAndWrite    : {
+			type: String,
+			index: {
+				unique: true,
+				partialFilterExpression: {'tokens.readAndWrite': {$exists: true}}
+			}
+		}
 	tokenAccessReadOnly_refs         : [ type:ObjectId, ref:'User' ]
 	tokenAccessReadAndWrite_refs     : [ type:ObjectId, ref:'User' ]
 	overleaf          :