Rebase Tor-Browser so there is a clear dividing line between non-tor specific features and tor specific features. Updates to tor-browser-build to build 'Base Browser' without tor, tor-launcher, PTs, etc.

The deliverable is a MVP of a base browser based on ESR 91. It will include:

• security/privacy/fingerprinting enhancements (proxy bypass fixes, purged telemetry, letter-boxing, various anti-fingerprinting measures, etc)
• no tor, no mention of TPI, no mullvad branding, etc
• vanilla Firefox strings/localization
• working Linux testbuilds, best-effort on Windows, macOS testbuilds
• no in-browser updating
• mullvad-browser testbuild target in tor-browser-build

The following phases are:

• Phase 2: Privacy browser.
• Phase 3: Migration to ESR 102.
• Phase 4: Browser release management.