Validate settings when importing and warn the user in case of error

[Cordarion the Grey]

While importing, invalid JSON fields (with e.g. syntax errors or schema errors) silently get rejected.

[Erinome]

How come that a user wants to import valid JSON-packed settings list which contains invalid JSON subfields? If the whole settings text got broken (i.e. due to wrong copy-paste boundaries) it will be rejected with a warning. And if a subfield has broken JSON inside it then it was probably exported this way and was broken anyway.

[Cordarion the Grey]

He exported his settings, used an external text editor to write a bunch of JSON objects, but forgot a comma somewhere, and tried to import back.

Adding a dozen of blacklist entries with current interface is a pain.

[Erinome]

Current interface allows export/import of a single setting. Also it has a (working) confirmation dialog showing a message whether an import was successful.

However currently it just fails with an error on a broken JSON - it will be fixed.

[Cordarion the Grey]

The issue still exists when importing all settings (through a form at the bottom of the page, not lightboxes).