Add cancel option to ReadCsvs command

[CodeCricut]

When a ReadCsvs command is issued, the user may want to cancel the command before it has completed. To fulfill this, these basic steps need to be taken:

• add UI for cancelling the command
• use a cancellation token within the ReadCsvsHandler command handler
• in the case of a cancellation, the changes should not be saved to the repositories within the Handle method of the command handler

Special considerations:

• if the command has already cleared the data in order to make way for the new data, the handle method should reload the previous data