Relax cookie restrictions to 'lax' to allow frontend to work

Our frontend requests the `/me` pathname in order to determine whether
it's logged in or not. Due to the fact that the sameSite attribute of
the session cookie was set to `strict` in a previous commit, the session
token was no longer sent along with HTTP calls initiated by JS. This is
due to the RFCs definition of "safe" HTTP calls in RFC7231.

The bug triggers the UI to show up like an unauthenticated user, even
after a successful login. In order to debug it a look into the send
cookies to the `/me` turned out to be very enlightening.

The fix this patch implements is rather simple, it replaces the sameSite
attribute to `lax` which enables the cookies for those requests again.

Some older and mobile clients were unaffected by this due to the lack of
implementations of sameSite policies.

References:
https://tools.ietf.org/html/rfc7231#section-4.2.1
https://tools.ietf.org/html/draft-ietf-httpbis-rfc6265bis-05#section-5.3.7.1
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie/SameSite
e77e7b165a

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

app.js

@@ -147,7 +147,7 @@ app.use(session({
   rolling: true, // reset maxAge on every response
   cookie: {
     maxAge: config.sessionLife,
-    sameSite: 'strict',
+    sameSite: 'lax',
     secure: config.useSSL || config.protocolUseSSL || false
   },
   store: sessionStore