See also:
#5: Product name and directory customization
#12: Create new branding directories and integrate Mullvad icons+branding
#14: Remove Default Built-in bookmarks
#35: Add custom PDF icons for Windows builds
#48: Replace Mozilla copyright and legal trademarks in mullvadbrowser.exe metadata
#51: Update trademark string
#104: Update shipped dll metadata copyright/licensing info
#107: Add alpha and nightly icons

MB 244: Fix the link to Mullvad Browser Support

We install a few addons from the distribution directory, but they are
not automatically enabled for PBM mode.
This commit modifies the code that installs them to also add the PBM
permission to the known ones.

Also:
#57: Purge unneeded about: pages

MB 243: Make sure about:mullvad-browser is treated as a new tab page

This commit disables the about:login page and removes the "Login and
Password" section of about:preferences.

We do not do anything to the real password manager of Firefox, that is
in toolkit: it contains C++ parts that make it difficult to actually
prevent it from being built..

Finally, we modify the the function that opens about:login to report an
error in the console so that we can quickly get a backtrace to the code
that tries to use it.

Windows and Linux will be distributed only as portable apps at the
beginning, so they should not be settable as default browsers.
We will need to improve the logic once we decide to ship system-wide
installers, too.

MB 71: Set the updater base URL to Mullvad domain

about:preferences allow to override some of our defaults, that could
be fingeprintable or have some other unwanted consequences.

Besides disabling the "Delete on close checkbox", disable also the
"Manage Exceptions" button when always using PBM.