Fix multiple fonts not compiling in elemental

Explicity define as string when passing value in the elemental default
theme

If not explicitly defined as string next variable assignment with not
maintain other fonts after the first one.
Add fallback in Elemental for blue sky preset

Add fallback for Helvetica Font in blue sky preset for the Elemental
theme
Merge branch 'fix_elemental_fonts' of https://github.com/mkly/concrete5-5.7.0 into mkly-fix_elemental_fonts
Merge branch 'develop' of github.com:concrete5/concrete5-5.7.0 into develop