Add slack in the other direction as well

What does this MR do (summary)?

Fix api key login

What changes have you made (detailed)?

Allow also EXPIRY_TIME seconds of slack in the other direction; now a server with time 1 second off is already breaking login.

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.

src/main/java/nl/tudelft/labracore/security/api/APIUserDetailsService.java

@@ -126,7 +126,8 @@ public class APIUserDetailsService implements
 			String apiSecret) {
 		long currentTime = now().toInstant(ZoneOffset.UTC).toEpochMilli();
 
-		if (!(details.getTimestamp() <= currentTime && currentTime <= details.getTimestamp() + EXPIRY_TIME)) {
+		if (!(details.getTimestamp() <= currentTime + EXPIRY_TIME
+				&& currentTime <= details.getTimestamp() + EXPIRY_TIME)) {
 			throw new CredentialsExpiredException(
 					"Expiry time reached: " + details.getTimestamp() + ", current: " + currentTime);
 		}