Owncloud speed

The default owncloud web ui is unpleasantly slow. These commits make it somewhat faster:

- We add an expires header to static content of 2 weeks, which allows the browser to cache static content.
- We override PHP's expires header so that it's not [set to 1981](http://stackoverflow.com/questions/8194481/why-is-expires-1981), which helps with caching.
- We compress responses with gzip.

This update also fixes some problems with the owncloud tasks:

- it doesn't update the cache when installing the owncloud package, since updating takes forever and should be done by another role.
- It adds a previously-missing restart notification for when the owncloud apache configuration changes.