Move giant gsb-threats.db to XDG_CACHE_HOME (among other changes)

Google Safe Browsing database (gsb-threats.db) is not 200 MB of user configuration, but a locally cached database file that can be redownloaded and recreated on demand. It belongs in XDG_CACHE_HOME and not XDG_CONFIG_HOME as it is now.

Should be just one change in https://gitlab.gnome.org/GNOME/epiphany/blob/318a8ad17b7bcb66668a63ce3eef772f41c1df3f/embed/ephy-embed-shell.c#L564 and some migration code to delete the database from XDG_CONFIG_HOME.

(.config is for user configuration, .local is for user-created data, .cache is for non-unique re-creatable data such as files downloaded from the web)

Most backup software will exclude XDG_CACHE_HOME by default and 200 MB of malware and phishing lists isn’t something people normally want to backup.