Implement Imports

[nplhse]

Is your feature request related to a problem? Please describe.
The project needs Imports that represent the file that is the origin of the data (e.g. the Allocations) that have been imported.

Describe the solution you'd like
This entity should:

• refer to a Hospital and to the related Allocations
• refer to a file that is located in the storage
• give the User the ability to give it a custom name
• include some data about the related Allocations (e.g. the total count, the times it has been re-imported, the count of rows that have been skipped during import)

Describe alternatives you've considered
There is no alternative.

Additional context
Some parts do not have to be included at this point, because the "real" import will follow later during Iteration 4 or 5.