If the old experience CSV importer is still running in the background, stop the user from starting the new one

Steps to Reproduce:
1.) Begin an import with 700 rows from a CSV file,
2.) Run Import
3.) For any reason decide to click on Stop Import and observe the importer is stopped.

Expected Results:

I can start a new import with 500 rows, and it will succeed.

Actual Results:

When I try to run the new import, it immediately shows a success message stating that the 500 records were imported, but the log file shows the last stopped import. No data has been imported to Jira.

Suggested change:

If the current process is still running in the background, prevent the user from starting a new one until the previous import is fully stopped. Please provide a message indicating how long the importer will be unavailable before a new import can begin.