Remove "layout-breve" class from widget template

[verythorough]

The "layout-breve" class adds a margin around the content of the widget iframe, adding awkward and unnecessary whitespace:

Considering that sites will generally style the iframe itself to conform to the desired layout, "layout-breve" seems undesireable in this context. Without it, the cards fill the iframe space, making the left margin an vertical whitespace pattern match with the rest of the page:

The one downside is that there is no margin between the cards and the vertical scrollbar. Setting the containing ul width to 99% via inline style would fix this, though it is a little hacky. (Ideally, this would be solved by making a lightweight widget-specific style list to replace the 6+ stylesheets currently being loaded in the iframe, but that's an issue I'll file separately.) If there are no objections to this, I'll make these changes and submit a PR.