Device groups handling improvements

The epic is about improving device groups handling in general.

Currently, we have static and dynamic groups.

Dynamic group is a filter with the name.

Static group is a group of devices with the same "group" attribute.

There are some issues around static and dynamic groups:

• the name of the static group can be the same as the name of the dynamic group;
• no explicit static group (removing the last device from the group means removing the group!)
• a device can only be in one static group at the time (this was intentional to avoid accidents - e.g. device being in both dev & prod groups, but turned out to be too restrictive)
• different handling of the static and dynamic groups in the system (e.g. deployments, UI)

One of the ideas is to unify static and dynamic groups somehow. e.g. the dynamic groups can remain the same, but the static groups may become simple dynamic groups (based on tags?).

There are some UX implications, especially when it comes to deployments, but I believe the idea is worth discussing.

It also means introducing simple dynamic groups to OS.