Convert MOZILLA_OFFICIAL conditionals to MC_OFFICIAL where needed.

- `--enable-official-branding` implies `MC_OFFICIAL` (no need to specifically set it)
- `--enable-official-vendor` can be used to set `MC_OFFICIAL` on builds without `--enable-official-branding` that should still be considered official release versions.
- `MC_OFFICIAL` implies `--enable-release`, meaning `DEVELOPER_OPTIONS` isn't set
- `MC_OFFICIAL` makes `nsXULAppInfo.getIsOfficial` return `true`
- `MC_OFFICIAL` makes `AppConstants.MOZILLA_OFFICIAL` (for compatibility in extensions) and `AppConstants.MC_OFFICIAL` return `true`
- Optional, for the time being: `MOZILLA_OFFICIAL` is still present in some places in case someone wants to build a Mozilla-alike official application and has the rights and necessary keys to use Mozilla-official third-party services. This must always be combined with `MC_OFFICIAL` to have a sane combination of defines. This may be removed in the future.

python/mozbuild/mozbuild/mozinfo.py

@@ -90,7 +90,7 @@ def build_dict(config, env=os.environ):
     d['bin_suffix'] = substs.get('BIN_SUFFIX', '')
     d['addon_signing'] = substs.get('MOZ_ADDON_SIGNING') == '1'
     d['require_signing'] = substs.get('MOZ_REQUIRE_SIGNING') == '1'
-    d['official'] = bool(substs.get('MOZILLA_OFFICIAL'))
+    d['official'] = bool(substs.get('MC_OFFICIAL'))
     d['sm_promise'] = bool(substs.get('SPIDERMONKEY_PROMISE'))
 
     def guess_platform():