stray racks and stray containers

[graik]

Racks can be created without any parent location. Likewise, Containers can be created without parent rack (according to data model). Stray racks and containers are more or less supported by the code but lead to oddities and subtle failures in links and filtering.

For example: In Container or Sample table, racks that have no location, cannot be filtered for.
Similar is probably true for containers without racks.

Solution:

• make Location a required field of Rack
• make Rack a required field of Container