Opt in to some of the NIGHTLY_BUILD features

The NIGHTLY_BUILD flag is used in mozilla-central to enable some useful developer and debugging features.

However, it is not set when building tor-browser because it is based on "a1" being part of version.txt.

Rather than switching on the NIGHTLY_BUILD flag ourselves and enabling everything, we could add our own flag TOR_BROWSER_NIGHTLY_BUILD and use it to selectively opt in to some of the features. We can base this on the update channel, but unlike MOZ_UPDATE_CHANNEL, we this will just be a binary flag so can be used in the C++ preprocessors.