Added Alert-Groups (Backend)

[f0o]

This PR adds Alert-Groups.

Alert-Groups are created in the table alert_groups. It expects a name and a desc. Both can be any kind of text.

A device is added to the group by adding an entry into the device_attribs table using alert_group for attrib_type and the group's alert_groups.id for attrib_value.

To maintain overview, the column alert_rules.device_id has been renamed to alert_rules.target.

To apply a rule to an alert-group, set alert_rules.target to alert_groups.id prefixed with a g.
Example: g1 for Group with alert_groups.id of 1.

This PR does not include any form of Frontend to populate the tables. This will be done separately.

Note: Because this PR renames a widely used column it requires indepth testing and revision!
Please drop me some comments in case I missed some files that still depend on alert_rules.device_id !

(Related: #616)

[laf]

Running on my test install now.

[f0o]

GUI draft: master...f0o:issue-616-frontend

[f0o]

Reworking like discussed on IRC.