ChangeScreen2: Add key binding for Expand All/Collapse All buttons

eb924ff74a90bbeb37cd3856dae4e9325598656b inroduced one of the most wanted
features, that the whole community has been waiting for years. Let's add
key bindings for that too.

Change-Id: I9b159a9a877cf95edd1628329a9448de3a59fe21
(cherry picked from commit 79f675a5a52c0f7ec6943cfdb6760462d615a432)
Fix build errors

Commit e786675 cherry-picked from master caused the build to fail
because of a missing import and missing message string definition.

Add the missing import and message definition.

Change-Id: I2639816d86e232c67421c91b594281b9b0a9bb8f
Merge "ChangeScreen2: Add key binding for Expand All/Collapse All buttons" into stable-2.8
Document that `reindex` must be run after enabling the secondary index

If the index is enabled, but not built before Gerrit is started, Gerrit
fails to start with IllegalStateException.

Update the documentation to mention that the `reindex` program must be
run after enabling the index.

Change-Id: I6e5fb35c408cf315c222577b23e14656b9e76810
Add 'reindex' to the pgm help output.

Change-Id: I237c738dbbc6baf3e73c2de408e04bf2ea982e79