Convert some loadPreferences() to getUserSetting()

Summary:
Ref T4103. This doesn't get everything, but takes care of most of the easy stuff.

The tricky-ish bit here is that I need to move timezones, pronouns and translations to proper settings. I expect to pursue that next.

Test Plan:
- Grepped for `loadPreferences` to identify callsites.
- Changed start-of-week setting, loaded Calendar, saw correct start.
- Visited welcome page, read "Adjust Settings" point.
- Loaded Conpherence -- I changed behavior here slightly (switching threads drops the title glyph) but it wasn't consistent to start with and this seems like a good thing to push to the next version of Conpherence.
- Enabled Filetree, toggled in Differential.
- Disabled Filetree, no longer visible in Differential.
- Changed "Unified Diffs" preference to "Small Screens" vs "Always".
- Toggled filetree in Diffusion.
- Edited a task, saw sensible projects in policy dropdown.
- Viewed user profile, uncollapsed/collapsed side nav, reloaded page, sticky'd.
- Toggled "monospaced textareas", used a comment box, got appropriate fonts.
- Toggled durable column.
- Disabled title glyphs.
- Changed monospaced font to 18px/36px impact.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16004

src/applications/differential/controller/DifferentialRevisionViewController.php

@@ -375,18 +375,19 @@ final class DifferentialRevisionViewController extends DifferentialController {
     $crumbs->addTextCrumb($object_id, '/'.$object_id);
     $crumbs->setBorder(true);
 
-    $prefs = $viewer->loadPreferences();
-    $pref_filetree = PhabricatorUserPreferences::PREFERENCE_DIFF_FILETREE;
+    $filetree_on = $viewer->compareUserSetting(
+      PhabricatorShowFiletreeSetting::SETTINGKEY,
+      PhabricatorShowFiletreeSetting::VALUE_ENABLE_FILETREE);
+
     $nav = null;
-    if ($prefs->getPreference($pref_filetree)) {
-      $collapsed = $prefs->getPreference(
-        PhabricatorUserPreferences::PREFERENCE_NAV_COLLAPSED,
-        false);
+    if ($filetree_on) {
+      $collapsed_key = PhabricatorUserPreferences::PREFERENCE_NAV_COLLAPSED;
+      $collapsed_value = $viewer->getUserSetting($collapsed_key);
 
       $nav = id(new DifferentialChangesetFileTreeSideNavBuilder())
         ->setTitle('D'.$revision->getID())
         ->setBaseURI(new PhutilURI('/D'.$revision->getID()))
-        ->setCollapsed((bool)$collapsed)
+        ->setCollapsed((bool)$collapsed_value)
         ->build($changesets);
     }