R1:661f077bf700

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.