Add warning to Customizer about Firefox bug; refs #15016

[skip sauce]
Merge pull request #15395 from twbs/firefox-customizer-warning

Add warning to Customizer about Firefox bug
feat(blocks): releasing new modules
style(glyph): rename glyphicon to glyph
feat(glyph): removing old glyph block
feat(dist): releasing first bem bootstrap dist
refactor(blocks): refactoring existing blocks into bem blocks