Fix table-layout emails, blacklist transforms by sender domain

Fix Twitter emails by overriding any 'table-layout' style when
overriding table width to 'auto'.

Add infrastructure to blacklist transforms by sender domain.
Not actually used yet, because the Twitter email fix above is more
generally useful and doesn't need to be special-cased.

Bug: 8403536
Bug: 8685326
Change-Id: I9e72e8e6336c138674839724df57eef1f86a2d9b
5 files changed