CSS adjustments in authentication flow and sidebar

A handful of (mostly trivial) CSS adjustments to smooth over some rough edges in the authentication workflow and bucket view:

 - Adds some padding to the notifications that are displayed when a login attempt fails.
 - Gets rid of the two-part animation that was happening when a nested comment thread was expanded or contracted. Now, the change is just manifested instantaneously.
 - Replaces the dotted border on the +/- expand/contract buttons with a solid outline, which prevents some weird visual artifacts that were occurring at the corners of the boxes.
 - Makes the little red menu with Reply/Share/More links appear immediately when the cursor enters the container for the comment, which feels a bit more responsive.
 - Adds ```cursor: w-resize``` to the "quotation" icon at the top of the collapsed sidebar, so as to mirror the ```ew-resize``` cursor used for the drag handle on the expanded bucket.