Add background job to abandon inactive changes automatically

Add a change cleanup job that runs periodically in the background.
This cleanup job can automatically abandon open changes that have been
inactive for a defined time.

Abandoning old inactive changes has a few advantages:
- it reduces the load for recomputing the mergeability flag when a
  change is merged
- it keeps dashboards clean
- it signals change authors that changes are considered outdated

Change-Id: Ia798667901fe8d734ca29bcf81c7b9b4a1eb4c50
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/Documentation/user-change-cleanup.txt b/Documentation/user-change-cleanup.txt
new file mode 100644
index 0000000..e569f38
--- /dev/null
+++ b/Documentation/user-change-cleanup.txt
@@ -0,0 +1,28 @@
+= Gerrit Code Review - Change Cleanup
+
+Gerrit administrators may configure
+link:config-gerrit.html#changeCleanup[change cleanups] that are
+executed periodically.
+
+[[auto-abandon]]
+== Auto-Abandon
+
+This cleanup job automatically abandons open changes that have been
+inactive for a defined time.
+
+Abandoning old inactive changes has the following advantages:
+
+* it signals change authors that changes are considered outdated
+* it keeps dashboards clean
+* it reduces the load on the server (for open changes the mergeability
+  flag is recomputed whenever a change is merged)
+
+If a change is still wanted it can be restored by clicking on the
+`Restore` button.
+
+GERRIT
+------
+Part of link:index.html[Gerrit Code Review]
+
+SEARCHBOX
+---------