redirect to auth callback if session / realm aren't found in memory

[shellhazard]

Problem
If the server restarts after a user authenticates, when they revisit a protected page they'll be greeted with an unfriendly 401 unless their cookie has already expired. The workaround for this is to have a user clear their cookies, but this is less than ideal.

Solution
When a user provides a token with an invalid session or realm, redirect them to the Discord authentication page instead of completely blocking them out.

Additional changes
While I was poking around, I figured I'd add configuration for the name of the cookie that gets set in the browser by this plugin as allowing only the default exposes an implementation detail, which is better avoided where possible.

EDIT: Still not working as intended, following up with maintainer

[VeryStrongFingers]

I ended up resolving this via 1b3219f

the issue turned out to be JWT changing signing key on every server start, the cookie was not the issue.
Discord application ID hashed is now the signing key to avoid this issue

still appreciate the PR and issue report though 😄