Pin sass install to work around sass/sass#1476

sass==3.4.6 has introduced a regression in the parsing of
pseudo-selectors such as

    :not(:focus)

which breaks our build. We need to temporarily pin to 3.4.5 to avoid
spurious build errors all over the place.
Add more usernames to blacklist

I noticed that root and postmaster were missing, one thing led to
another, and I added a bunch of names from this list:

    https://programmers.stackexchange.com/questions/115425/is-there-a-list-of-common-usernames-to-reserve-in-a-new-system