R1:e281c5ee9045

Security - disable conduit act as user by default

Summary: Introduce a new configuration setting that by default disables the conduit as as user method. Wordily explain that turning it on is not recommended. Fixes T3818.

Test Plan:
```
15:25:19 ~/Dropbox/code/phalanx/src/applications/conduit (T3818)
~> echo '{}' | arc call-conduit --conduit-uri http://phalanx.dev/ user.whoami
Waiting for JSON parameters on stdin...
{"error":null,"errorMessage":null,"response":{"phid":"PHID-USER-tghb3b2gbdyezdcuw2or"…