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","userName":"btrahan","realName":"Bob Trahan","image":"http:\/\/phalanx.dev\/file\/data\/yncjbh7phk7ktrdhuorn\/PHID-FILE-qyf4ui3x2ll3e52hpg5e\/profile-profile-gravatar","uri":"http:\/\/phalanx.dev\/p\/btrahan\/","roles":["admin","verified","approved","activated"]}}
15:25:34 ~/Dropbox/code/phalanx/src/applications/conduit (T3818)

<go edit libconfig/conduitclient to spoof another user...>

~>  echo '{}' | arc call-conduit --conduit-uri http://phalanx.dev/ user.whoami
Waiting for JSON parameters on stdin...
{"error":"ERR-CONDUIT-CORE","errorMessage":"ERR-CONDUIT-CORE: security.allow-conduit-act-as-user is disabled","response":null}
15:26:40 ~/Dropbox/code/phalanx/src/applications/conduit (T3818)

<enable option via bin/config....>

~>  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-6lcglnzbkiamdofishgi","userName":"xerxes","realName":"Xerxes Trahan","image":"http:\/\/phalanx.dev\/file\/data\/n2kyeevowetcuynbcxrg\/PHID-FILE-voquikectzpde256zzvm\/profile-1275455993.jpg","uri":"http:\/\/phalanx.dev\/p\/xerxes\/","roles":["verified","approved","activated"]}}
```

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: jevripio, sowedance, epriestley, Korvin

Maniphest Tasks: T3818

Differential Revision: https://secure.phabricator.com/D9881