Make id/phid Conpherence stuff more consistent

Summary:
Currently, `selected_conpherence_id` is actually a PHID. Instead, use ids everywhere.

Also, use replaceState instead of pushState. This means "back" takes you out of conpherence (not back to the last thread you looked at) but I think that's more consistent with user expectation.

Test Plan: Loaded thread list, loaded specific thread.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5503