Store integer settings as integer

[zunda]

This change introduces INTEGER_KEYS for the settings with values to be stored as Integer.

Without typecasting the retention period settings to Integer, they are stored as String:

>  SELECT var,value FROM settings WHERE var LIKE '%retention_period';
              var               |  value
--------------------------------+----------
 media_cache_retention_period   | --- '14'+
                                |
 backups_retention_period       | --- '7' +
                                |
 content_cache_retention_period | --- ''  +
                                |
(3 rows)

This makes app/models/content_retention_policy.rb detect the settings as unset:

$ rails c
> Setting.media_cache_retention_period
=> "14"
> Setting.media_cache_retention_period.is_a?(Integer)
=> false
> ContentRetentionPolicy.current.media_cache_retention_period
=> nil