Adds COMPRESS_OFFLINE option to local_settings

The Ubuntu Havana packages includes pre-compressed
assets and does not install a package that provides
lesscpy.

This re-adds COMPRESS_OFFLINE=True to utilize this
and avoid a broken Horizon install on Ubuntu.

Change-Id: I6671938a87744a4dd0b84021ca87cd49aea10741
Closes-Bug: #1272441