Add full LESS command to compile_css error display for debugging
Use existing LESS variables in circles theme

There were some redundant variables in the variables.less file for the
circles theme.  Use the already existing variables (which show up in the
"Sidebar" and "Navbar" sections of the theme editor) where possible.
This reduces the number of "Advanced" variables for this theme, and
hopefully also reduces confusion.
Harmonize administrative links provided in each theme
Make theme editor URLs work with or without slashes
Add theme Node.js and LESS install instructions to docs.txt