Combine mullvad fluent files into one.

Part of tpo/applications/mullvad-browser#349 (closed). Should only land after tpo/applications/mullvad-browser!141 (merged) lands and we get confirmation from @pierov about what needs to change in the build system.

Copied the en-US file from tpo/applications/mullvad-browser!141 (merged). For the other locales I just merged the files together and dropped unused about-mullvad-browser-page-title.