Set up L2TP VPN on EdgeRouter

The instructions on Unifi's official site are "feature complete" - we use these instructions with no modifications to configure the L2TP VPN for EdgeRouters: https://help.ui.com/hc/en-us/articles/204950294-EdgeRouter-L2TP-IPsec-VPN-Server

Also:

1. Store all credentials in the PCRT Group: the PSK and each VPN user.
2. When configuring the client on Windows, make sure to do the step for checking the "MS CHAP v2" box in VPN adapter properties.
3. Configure a DNS or DDNS entry so users can access a "pretty url" instead of the raw WAN IP (i.e. company-name.clients.mynet.care vs 123.123.123.123)