mediawiki: use mods-enabled, prepare for HAT

We move the whole management of all apache modules to the standard
debian location, mods-enabled, and out of the apache2 main config file,
that becomes much shorter with this modifications.

We also include modules depending on whether we're on a HAT installation
or on a standard mod_php5 installation.

This change needs PatchSet 147086 to be merged in order to work.

Change-Id: Ia463120718dceab087ad3f8e3f35917fa879f387
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>