Add Discourse (self-hosted)

[riking]

I was going to submit a PR, but wasn't sure about the format for "working on it".

Name: Discourse
url: https://www.discourse.org/
image: https://d11a6trkgmumsb.cloudfront.net/optimized/3X/b/3/b33be9538df3547fcf9d1a51a4637d77392ac6f9_2_32x32.png

if above link is dead, view source meta.discourse.org and pull the <link rel="icon" type="image/png link

Supports TOTP
U2F is planned: https://meta.discourse.org/t/yubikey-u2f-key-support/88543/26

Documentation: https://meta.discourse.org/t/discourse-2-0-0-beta4-release-notes/82446
and https://meta.discourse.org/t/two-factor-backup-codes/90935