Use a filename with a date-time portion when backing up uBlock data

[gorhill]

Noticed today the file picker was gone when exporting uBlock data (got lost somewhere in there).

So rather than have the browser save to ublock-backup.txt, ublock-backup (1).txt, ublock-backup (2).txt, etc., add a date-time part to the filename so that the browser does not rename using a less than useful naming scheme.

[gorhill]

Fixed with 52e8cb5.