migrate ::imagescaler -> ::mediawiki::multimedia

* manifests/imagescaler.pp -> modules/mediawiki/manifests/multimedia.pp
* rename 'removetmpfiles' cron job to 'clear_imagemagick_temp_files'
* move clear_imagemagick_temp_files command to a template file
* alphabetize packages and ensure => present rather than ensure => latest
* File['/etc/wikimedia-image-scaler']: just touch an empty file rather than
  write contents to make intent clearer and to avoid the impression that the
  content is significant.
* drop unused Monitor_group['imagescaler'] (there's a monitor_group for scalers
  that is qualified with a cluster name in manifests/role/mediawiki.pp).

Change-Id: I28bcbab762e26e2dd5513b605aad28a29cf4ac16