Render timezone names more readably, with spaces rather than underscores ("America/Los Angeles", not "America/Los_Angeles").

Summary:
See downstream https://phabricator.wikimedia.org/T902. Currently, timezones are rendered with their raw internal names (like America/Los_Angeles) which include underscores.

Replacing underscores with spaces is a more human-readable (and perhaps meaningfully better for things like screen readers, although this is pure speculation).

There's some vague argument against this, like "administrators may need to set a raw internal value in phabricator.timezone and this could mislead them", but we already give a pretty good error message if you do this and could improve hinting if necessary.

Test Plan: Viewed timezone list in Settings and the timezone "reconcile" dialog, saw a more-readable "Los Angeles".

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20559