Improve consistency of global LESS variable declaration in Flamingo

All the XWiki specific variables should be defined in variables.less . Other LESS files should only contain styles in themselves.

In addition, the variable `@loginFormMaxWidth` should be renamed because it did not respect our naming convention (strictly speaking we don't have one for naming LESS variables, but we have one for HTML/CSS classes and bootstrap had one that we followed for pretty much all other variables). This is technically a backward compatibility break, but the name and use of this variable is so niche that I doubt anyone used it in a customization...