Add notification for async import finished event

[luka-nextcloud]

• Resolves: Add asynchronous import notification #2764

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stableX.X
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[samin-z]

ok so what i mentioned is already covered, i only found two things which one is not regarding this pr but thought maybe worth mentioning, and the other one could useful:

1. when import is done (specially if the import is very big) in the activity table per each row that is inserted there is a message which for this case can get a bit out of hand, but we can tackle that maybe in another task.

2. worth mentioning, if the import fails for whatever reason there is no notifications, i created a table and imported a file, while the background job was starting i removed the file, but no notifications was returned for the import that it failed. i also tried it with removing the table.