Implement a scope selector for the global search

Summary:
See M1433. Fixes T7266. Fixes T4475. Ref T7314.

Future work/notes/etc:

  - Write the User Guide (see TODO).
  - This might needs some design tweaks -- I think it's functionally almost-equivalent to the mock, but the UI isn't quite the same.
  - (Mobile design is a touch off-looking I think?)
  - When you use a custom query, the duplicate "magnifying glass" icons are a little weird. Maybe change one or the other.
  - Maybe worth adding an "Open Documents in Current Application" option? Planning to wait for feedback on that.
  - Need a Quicksand integration to change the current application at some point.
  - Searching in "Current Application" from, e.g., the 404 page just searches all documents. Current plan is to just document this behavior, since the icon is a pretty good callout and it seems plausible that this is intuitive enough that users won't have a hard time with it.

Test Plan:
New dropdown:

{F379150}

Device-ish:

{F379151}

Normal search (current application, from maniphest, selects tasks):

{F379153}

Application search from non-application:

{F379154}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: johnny-bit, epriestley

Maniphest Tasks: T7266, T7314, T4475

Differential Revision: https://secure.phabricator.com/D12509

src/applications/phriction/application/PhabricatorPhrictionApplication.php

@@ -69,4 +69,10 @@ final class PhabricatorPhrictionApplication extends PhabricatorApplication {
     return 0.140;
   }
 
+  public function getApplicationSearchDocumentTypes() {
+    return array(
+      PhrictionDocumentPHIDType::TYPECONST,
+    );
+  }
+
 }