Implement user dashboard view

This changes the way the change list view is layed out (flexbox
instead of tables) for a couple reasons:

+ Table elements (td, tr, etc.) cannot be arbitrarily appended
  to just any DOM node, which caused breakage when I originally
  attempted to use a <content> tag to bring in the header or
  group title content into gr-change-list-item.
+ The flexibility of flexbox allows us to style the change list
  much easier on smaller screens, so it was probably going to
  happen anyway.
+ Full-width (colspan="<total number of columns in the table>")
  rows for the headers is much more difficult when using elements
  outside of the gr-change-list-item brought in via <content> as
  described above.
+ gr-change-list-item was doing too much anyway.

Feature: Issue 3700

Change-Id: I6536bf7d18adfa460507f8050a15d83e84af82a7

polygerrit-ui/app/scripts/changes.js

@@ -25,6 +25,13 @@ Changes.DiffType = {
   REWRITE: 'REWRITE',
 };
 
+Changes.Status = {
+  NEW: 'NEW',
+  MERGED: 'MERGED',
+  ABANDONED: 'ABANDONED',
+  DRAFT: 'DRAFT',
+};
+
 // Must be kept in sync with the ListChangesOption enum and protobuf.
 Changes.ListChangesOption = {
   LABELS: 0,