Pregeneration of content cache for specific users

Title: Pregeneration of content cache for specific users

Documentation:
The pregeneration of content cache has been enhanced to allow the site
administrator to choose which users the content cache should be made for.
This can be used to prepare new content for certain users/usergroups.
The default setting for this is to generate it for anonymous users.

Configuration:
To configure this the variable PreviewCacheUsers setting in site.ini is used.
It is an array with user IDs of which should be used. The system will then use
the assigned roles for that user to generate the content cache file.

It is also possible to use some special identifiers as entries, these are supported:
- anonymous - Means to fetch the user which is defined as anonymous users
- current - Means to use the user which is publishing the object

If you intend to create cache for a given user group you should choose one specific user
from the group. For instance you could create a dummy user for this purpose.

Example:
Here we generate cache for anonymous, the current user and a specific user.

PreviewCacheUsers[]
PreviewCacheUsers[]=anonymous
PreviewCacheUsers[]=current
PreviewCacheUsers[]=86