Reduce visibility of "Host" and "Path" Differential fields by default

Summary:
See discussion in T838. These fields expose information which it isn't necessary
or useful to expose in the general case.

• Disable fields by default, allow them to be enabled in config (these fields

were useful for me at Facebook when I had access to all the machines).

• Remove 'sourcePath' from Conduit methods other than differential.query.
• Condition 'sourcePath' field in Conduit on the caller being the revision

author. This is a bit hacky but not so awful.

Test Plan:

• Verified fields are gone by default and restored by configuration.
• Verified Conduit no longer returns these fields other than

differential.query.

• Verified field presence/absence according to authorship in

differential.query.

• Grepped around in arcanist to make sure we aren't relying on sourcePath.

There's a workflow in "arc merge" that technically might hit it, but I think
it's unreachable, definitely irrelvant (we never use source path as a
distinguisher under git/hg, and can't 'arc merge' in SVN) and it's going away
Real Soon Now anyway.

Reviewers: btrahan, arice

Reviewed By: arice

CC: aran, epriestley

Maniphest Tasks: T838

Differential Revision: https://secure.phabricator.com/D1582