Draft: Combine mullvad fluent files into one.
Part of tpo/applications/mullvad-browser#349. Should only land after tpo/applications/mullvad-browser!141 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. For the other locales I just merged the files together and dropped unused about-mullvad-browser-page-title
.