ESR 128 Patch Review (Desktop)

• cc @boklm @brizental @clairehurst @dan @henry @jwilde @ma1 @pierov
• cc @bella @ruihildt
• cc @manuel

PieroV's ESR-128 Rebase MR: !130 (merged)

This is the Mullvad Browser equivalent to tor-browser#42616 (closed), so go there for instructions please and thank you 😄

Patches to Review

• Bug 290: Add default bookmarks in alpha channel for testing
  • reviewed by:
• MB 80: Enable Mullvad Browser as a default browser
  • reviewed by:
• MB 234: Disable OS spoofing in HTTP User-Agent.
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• MB 214: Enable cross-tab identity leak protection in "quiet" mode
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• MB 213: Customize the search engines list
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• MB 188: Customize Gitlab Issue and Merge templates
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• MB 163: prevent uBlock Origin from being uninstalled/disabled
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• MB 160: Disable the cookie exceptions button
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• MB 34: Hide unsafe and unwanted preferences UI
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• MB 79: Add Mullvad Browser MAR signing keys
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• MB 112: Updater customization for Mullvad Browser
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• MB 21: Disable the password manager
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• MB 47: uBlock Origin customization
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• MB 97: Remove UI cues to install new extensions.
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• MB 39: Add home page about:mullvad-browser
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• MB 37: Customization for the about dialog
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• MB 63: Customize some about pages for Mullvad Browser
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• MB 20: Allow packaged-addons in PBM.
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• MB 1: Mullvad Browser branding
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• MB 38: Mullvad Browser configuration
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