Remove subject IDs and improve request handling

server/asn1/StudySystemProtocol.asn1

@@ -6,7 +6,7 @@ ActivityType ::= ENUMERATED {
 }
 
 Session ::= SEQUENCE {
-	subjectId INTEGER,
+	subject UTF8String,
 	type ActivityType,
 	timestamp INTEGER,
 	minutes INTEGER
@@ -24,8 +24,7 @@ Request ::= SEQUENCE {
 }
 
 Activity ::= SEQUENCE {
-	subjectId INTEGER,
-	subjectName UTF8String,
+	subject UTF8String,
 	type ActivityType,
 	priority INTEGER
 }