Install svg from branding theme to browser/chrome/icons/default

In tor-browser-build!943 (comment 3013584), @pierov mentioned that we have svg files (added in commits Bug 2176: Rebrand Firefox to TorBrowser and MB 1: Mullvad Browser branding), but we currently don't install them to browser/chrome/icons/default.

If we do that we can then update the debian package to link them from /usr/share/icons/hicolor/scalable/apps.

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