Improve default block quote indentation.

Add default left indention to blockquote.

Bootstrap Reboot (which Guise Skeleton uses as a reset) in v4.1.3 doesn't provide a useful <blockquote> style:

Block quotes are usually left-indented as Wikipedia describes, and apparently in APA and MLA as well. See also How to Use Block Quotations in Writing.

Provide at least some left indentation, e.g.:

There's probably no need to provide CSS variables, as this is not a Guise Skeleton component; normal CSS selection and property setting should suffice.