temporary settings: allow editing without overwriting everything

Created by: limitedmage

Currently, the only way to update temporary settings for authenticated users is to overwrite the whole settings object. This can cause changing some settings to delete or revert unrelated settings if the settings have not been completely synced between two different devices/browsers.

With this change, editing settings can be done by merging the desired changes with the existing settings in the database, thus making temporary settings more robust.