fix unable to login if bad btUsername cookie

Ran across this bug when I disabled one account from another account. Was completely unable to login with any account because $_COOKIE['btUsername'] was not getting changed during the login process.

Testing revealed this to happen anytime $_COOKIE['btUsername'] was an invalid username or a disabled user.

Nasty bug. Took me like an hour to debug.

May only be partially fixed. I was stuck and ready to give up on this until I cleared all my cookies. Then I reproduced it by putting the invalid btUsername cookie back. Then my patch started working. Suggests that maybe the sessionID cookie was causing the username cookie to get reset or something.

src/login.php

@@ -67,6 +67,31 @@
 			$fail = false;
 			echo "
 				<script type='text/javascript'>
+					// Fakhruddin Ujjainwala, CC BY-SA 4.0, https://stackoverflow.com/a/24103596/3480193
+					function setCookie(name,value,days) {
+						var expires = \"\";
+						if (days) {
+							var date = new Date();
+							date.setTime(date.getTime() + (days*24*60*60*1000));
+							expires = \"; expires=\" + date.toUTCString();
+						}
+						document.cookie = name + \"=\" + (value || \"\")  + expires + \"; path=/\";
+					}
+
+					// When logging in, we need to overwrite any existing cookies, to prevent a bug where you can't log in if you got disabled while logged in.
+					// You can't set cookies in PHP after you've written any output.
+					// Therefore, we'll do it in JavaScript.
+					setCookie(
+						'btUsername',
+						'" . $_SESSION['btUsername'] . "',
+						'" . $COOKIE_EXP_TIME . "'
+					);
+					setCookie(
+						'btPassword',
+						'" . $_SESSION['btPassword'] . "',
+						'" . $COOKIE_EXP_TIME . "'
+					);
+
 					window.location = 'members/';
 				</script>
 			";