Allow to reconnect folder with pre-existing sync database

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## Feature request

**Which Nextcloud Version are you currently using:** Nextcloud Desktop Client Version 3.16.6 (Ubuntu)

**Is your feature request related to a problem? Please describe.**
I just reinstalled my OS and put back my files locally.
Steps that I took in the process

1. Back up sync folder to external drive (using `rsync`)
2. Wipe & reinstall OS
3. Copy back sync folder from external drive (using `rsync`)
4. Install Nextcloud Desktop
5. Authenticate in Nextcloud Desktop

* When trying to set up the folder connection, the UI complains that the folder already is used for synchronisation.
* When copy the folder configuration from the old config to the new config file (e.g. `/home/keunes/.config/Nextcloud/`), there is a warning that the same folder is synchronised by two users and that this is not supported (while I have changed my local username, my Nextcloud server username has remained the same).

I had to find and remove reminiscences of the old sync set-up from the sync folder (e.g. `.sync_27406f076ef0.db`). Now it's taking a long time time to check again every file (which I had hoped to prevent, given that we're talking thousands of files).

**Describe the solution you'd like**
Rather than refusing to set up the folder connection, offer to 'reconnect' the folder. It should be added using the existing local database and the local ignore list. The database should be checked and updated with the new folder root path.

**Describe alternatives you've considered**
Add a one-click button to remove the files that were used for the old synchronisation set-up, so that I don't have to go hunting for them manually.

**Additional context**
n/a