Make use of `unsubscriber` in the core app so that when someone clicks unsubscribe we do the unsub for them

[elie222]

If the unsubscribe is unsuccessful then we can have the user finish the flow themselves.

Also note that some unsub links are actually just to send an email, so we can skip using the unsubscriber for those.

PR for unsubscriber is here, but it's yet to be used in the web app.
#236

CC @madhav-relish

[elie222]

Important: the app should work without this as well. Because running it in this mode means running another service, it will be easier for self hosters to not have to use the unsubscriber if they don't want to.

This can be enabled via an env var: UNSUBSCRIBER_URL and if it's not set then it isn't used.

[elie222]

Most of the code for the task is here, but the code in that PR has not been tested. You can use it for reference (no need to build off of that PR):
#376