cubicweb #1988706 [facets] fix more ui / display issues [resolved]

and have less magic in the code (.py, .css)

prioritynormal
typebug
done in3.14.0
load1.000
load left0.000
closed by#ad0581296e2c [facets] try to get rid of arbitrary constants, be prettier and eat less space (closes #1988706)