support for using STS to redirect plaintext to TLS

[slingamn]

If I understood correctly, we're basically talking about integrating the functionality described here into Oragono itself: darwin-network/slash#5

1. Designate one or more plaintext listeners as "redirect-only"
2. When clients connect to them, Oragono serves an STS policy (via CAP LS) plus some human-readable text about how it's necessary to reconnect via TLS

[poVoq]

@slingamn could you please add to the manual how to configure this? It refers everywhere to the manual, but I can't find how to do it in the manual. Maybe I am just blind? Maybe the manual on the website needs to be updated?

Edit: ahh: https://github.com/oragono/oragono/blob/master/docs/MANUAL.md#how-can-i-redirect-users-from-plaintext-to-tls
Not on the website though...