style: in preparation for bootstrap, use bootstrap compatible button class names

Give all buttons a styling (default, success, danger, warning) and rename the sizes to sm and xs.

This is a subset of a bigger changeset. The subset was extracted by Mads Kiilerich, mostly by:

sed -i \
-e 's,btn btn-small,btn btn-default btn-sm,g' \
-e 's,btn btn-mini,btn btn-default btn-xs,g' \
-e 's,btn-default btn-\(xs\|sm\) btn-\(success\|danger\|warning\),btn-\2 btn-\1,g' \
-e 's,class_="btn",class_="btn btn-default",g' \
`hg mani`