Issues with Discord support

[jgoerzen]

Hi,

After the upgrade, I found I needed to:

• NOT set matrix_appservice_discord_auth_usePrivilegedIntents
• Also set:

matrix_appservice_discord_configuration_extension_yaml: |
        ghosts:
                usernamePattern: :username

when usePrivilegedIntents was set to true as documented in the docs directory, I was unable to get it to work with Discord.

Also, when the usernamePattern was the default, all users had #xxxx appended. Tab completion of @-mentions causes the message to fail to be sent to Discord, since Discord rejects it on "no such nick" grounds.

#748 no longer appears to be necessary thanks to the changes with mailer, and upstream appservice-discord says that the homeserver URL should be set to the public URL (though, confusingly, the example shows the local URL). Perhaps #748 should be reverted.