Implement "user" and "users" Maniphest custom fields

Summary:
Ref T2575. Implements "user" (zero or one users) and "users" (zero or more users) field types.

Also allows custom fields to participate in the handle pipeline.

Test Plan: {F35071}

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2575

Differential Revision: https://secure.phabricator.com/D5287

src/docs/userguide/maniphest_custom.diviner

@@ -44,8 +44,8 @@ Each array key must be unique, and is used to organize the internal storage of
 the field. These options are available:
 
   - **label**: Display label for the field on the edit and detail interfaces.
-  - **type**: Field type, one of **int**, **string**, **bool**, **select**,
-    **remarkup**, or **date**.
+  - **type**: Field type: one of **int**, **string**, **bool**, **select**,
+    **remarkup**, **user**, **users**, or **date**.
   - **caption**: A caption to display underneath the field (optional).
   - **required**: True if the user should be required to provide a value.
   - **options**: If type is set to **select**, provide options for the dropdown
@@ -54,8 +54,10 @@ the field. These options are available:
     show next to the checkbox.
   - **checkbox-value**: If type is set to **bool**, the value to show on
     the detail view when the checkbox is selected.
-  - **default**: Default field value. For **date**, you can use a string like
-    `"July 4, 1990"`, `"5PM today"`, or any other valid input to `strtotime()`.
+  - **default**: Default field value.
+    - For **date**, you can use a string like `"July 4, 1990"`, `"5PM today"`,
+      or any other valid input to `strtotime()`.
+    - For **user** and **users**, you can use an array of user PHIDs.
   - **copy**: When a user creates a task, the UI gives them an option to
     "Create Another Similar Task". Some fields from the original task are copied
     into the new task, while others are not; by default, fields are not copied.