Replace callsites to sha1() that use it to asciify entropy with Filesystem::readRandomCharacters()

Summary: See T547. To improve auditability of use of crypto-sensitive hash
functions, use Filesystem::readRandomCharacters() in place of
sha1(Filesystem::readRandomBytes()) when we're just generating random ASCII
strings.

Test Plan:

• Generated a new PHID.
• Logged out and logged back in (to test sessions).
• Regenerated Conduit certificate.
• Created a new task, verified mail key generated sensibly.
• Created a new revision, verified mail key generated sensibly.
• Ran "arc list", got blocked, installed new certificate, ran "arc list"

again.

Reviewers: jungejason, nh, tuomaspelkonen, aran, benmathews

Reviewed By: jungejason

CC: aran, epriestley, jungejason

Differential Revision: 1000