Migrate new identity and security level preferences in 11.5.8

We would like to get rid of all extensions.torbutton.*, at least for New Identity and Security Level, which will be used also in Base Browser/Privacy Browser.

Since we are forcing users to pass through 11.5.8 (#40678 (closed)), we could make an effort and create the migration code for this release, rather than keeping migration code for a long time.