avoid running lessc every time the application starts

If mod_wsgi is configured to recycle worker processes lessc could end up
running very frequently and it's quite expensive, so we should avoid it.

The admin will just have to work around the modtime issues when they are
updating their site theme. It's a bit sad, but given the way webassets'
modtime update checking currently works (it assumes mod times only ever
go forwards) there is no other simple solution.

Change-Id: I9750632f639584147603f3b45a0158af1f275038