Update file list header to a single row

Uses an enum to describe how many files are expanded:
- NONE
- SOME
- ALL

When NONE:
- Expand all button is visible
- Collapse all button is hidden
- View mode/Settings hidden

When SOME:
- Expand all button is visible
- Collapse all button is hidden
- View mode/Settings are visible
  - Selected view mode is highlighted in blue

When ALL:
- Expand all button is hidden
- Collapse all button is visible
- View mode/settings same as 'SOME'

Change-Id: I9c4f1e302ad94de6034b9cbaffffae649f0bc6b8

polygerrit-ui/app/template_test_srcs/template_test.js

@@ -23,6 +23,7 @@ const EXTERN_NAMES = [
   'GrDiffLine',
   'GrDomHooks',
   'GrEtagDecorator',
+  'GrFileListConstants',
   'GrGapiAuth',
   'GrGerritAuth',
   'GrLinkTextParser',