Do a cleanup of the NSIS script

The NSIS script could use a cleanup, such as better formatting etc etc.

Also, we have a draft for a system-wide installer, but we changed scope for that (not so long ago we decided to do user-only install, but still write to the registry to have the uninstall information and later add file associations/default browser). In general the Mullvad Browser installer will have several differences for which it might more sense to create a completely separate script rather than having a lot of [% IF system_installer %].

BTW, TIL the original installer was located at https://github.com/moba/tbb-windows-installer, if we want to credit it with a link (even though we should probably add credits also to the NSIS examples, at that scripts seems quite similar to some of them).