…oject#2235)

## Problem

The storage page shows an empty state (no devices found) after
activating/deactivating devices.

Steps to reproduce: 

* Go to iSCSI page and activate devices.
* Go back to storage.
* The storage page shows "No devices found" instead of the configured
devices and result.

## Solution

Source of the problem: the query for getting the available devices
requires the result of the query for getting all the devices, but it is
not refreshed when there are changes in the list of devices.

For solving it, the `useAvailableDevices` hook uses *useMemo* which
refreshes the list of available devices if any of the source data
changes (either *devices* or the list of *usable Devices*).

See
https://tkdodo.eu/blog/react-query-data-transformations#2-in-the-render-function:
"Especially if you have additional logic in your custom hook to combine
with your data transformation, this is a good option."

## Testing

* Tested manually