nested group support

We're using ansible to configure our servers and use a python script, that pulls information off the observium database for that task.
In order to group servers more effectively for hierarchical playbooks, I wanted nested groups.

I added support for nested groups in `includes\alerts.inc.php` (and I think I also solved the `// FIXME, what is this?` ).
I had to change the sql query a bit to allow for joins - that's why the patch affects that many lines - it actually isn't that big.

Usage:
In device rules simply add the following for example:
group includes vpnservers

One thing's still to do: You need to rebuild the groups again if one of the groups a include-rule relies on has changed after the include-rule has been updated.
I would've added it already but I'm not yet familiar with the codebase of observium yet and haven't found all the code that does the group building yet.
This is a start though