run apt-get autoremove after updating system packages

Old kernels can build up and some packages may not be needed anymore.

See https://discourse.mailinabox.email/t/storage-space-decreasing/2525/5.

CHANGELOG.md

@@ -13,6 +13,10 @@ Mail:
 
 * Updated IMAP/POP cipher list to Mozilla's current mondern recommendation.
 
+Installer:
+
+* We now run `apt-get autoremove` at the start of setup to clear out old packages, especially old kernels that take up a lot of space. On the first run, this step may take a long time.
+
 v0.24 (October 3, 2017)
 -----------------------
 

setup/system.sh

@@ -96,6 +96,12 @@ echo Updating system packages...
 hide_output apt-get update
 apt_get_quiet upgrade
 
+# Old kernels pile up over time and take up a lot of disk space, and because of Mail-in-a-Box
+# changes there may be other packages that are no longer needed. Clear out anything apt knows
+# is safe to delete.
+
+apt_get_quiet autoremove
+
 # ### Install System Packages
 
 # Install basic utilities.