Add @font-weight-base variable

Created by: ruiramos

Bootstrap resets the font properties of many of its elements (see #15925 (closed), etc), setting amongst other things the font-weight to normal (400).

This makes it very hard to use a different cross site default (in my particular case, needed the default for font-weight to be 300).

Could we solve this by using a @base-font-weight variable instead of the hard-coded normal value?

Thanks!