Fix Pepperminty Wiki login cookie

[sbrl]

Pepperminty Wiki uses a cookie to record when a user is logged in. This is normal, but apparently the behaviour of cookies is changing soon. See this error message from Firefox 79.0b9:

Cookie “PHPSESSID” will be soon rejected because it has the “sameSite” attribute set to “none” or an invalid value, without the “secure” attribute. To learn more about the “sameSite“ attribute, read https://developer.mozilla.org/docs/Web/HTTP/Headers/Set-Cookie/SameSite

[sbrl]

Uh-oh - it looks like only PHP 7.3+ supports setting the SameSite cookie flag with setcookie(). We've implemented a warning message via error_log() for users of PHP 7.2 and below.

Anyway, this issue is now fixed and will be released in v0.22.