Mitigate session expiry by using fallback session

Unfortunately, an expired session cookie wreaked havoc to our Advent of
Code commands: All commands that relied on leaderboard data failed
because we couldn't refresh our data and the cache had expired.

To mitigate an expired session, I've added a fallback session feature
that enables us to try again with a different session. While it will
issue an error message to inform us to refresh the expired session
cookie, it does mean that the functionality should continue to work in
the mean time.

The fallback session cookie is currently set to my session cookie, using
an environment variable, `AOC_FALLBACK_SESSION`. It is important that
the user connected to the session is a member of all boards and that
it's a fresh session: We don't want our fallback to expire!

At the same time, while a single fallback session works, the AoC website
also does not like too many requests from a single user. That's why
we'll still use a multi-session model under normal circumstances.

To check for expired sessions, I've added a URL check: The Advent of
Code website will silently redirect people with an expired session,
issuing an 200: OK status as usual. The only way to really check for it
is by comparing the final URL in the response object to the URL we set
out to GET. I've added a custom exception to signal such an unexpected
redirect.

Finally, instead of having the commands just break, I've added an
Exception signal that propagates back to the caller. The solution, with
try-except, is a bit hacky and could benefit from an actual error
handler, but I wanted to get things fixed first; polish can be added
later.

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