Enable verification of langpacks checksums

When switching to esr60 we temporarily disabled the checking of langpacks checksums in projects/firefox-langpacks/build because they were missing in the SHA512SUMS files.

It looks like the bug causing this issue will be fixed soon: https://bugzilla.mozilla.org/show_bug.cgi?id=1463749

We should enable this check again in the next release where this bug has been fixed.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information