Only use a single indentation level for tangents

Change I64c2569f2bd3aa092d57bec27b672d9528f925b7 added a small regression
where nested replies are indented multiple levels.  This adds a css rule
to override the left margin back to 0 for replies to tangents.

Change-Id: I1b0cb2e020cfa69f7ec792cb55c93d4c3a5f0b7e