[#5862] FxA button - append button_class to fallback buttons
[#5862] FxA button - append button_class to fallback buttons

hoosteeno/bedrock,flodolo/bedrock,pascalchevrel/bedrock,kyoshino/bedrock,sgarrity/bedrock,ericawright/bedrock,hoosteeno/bedrock,sgarrity/bedrock,pascalchevrel/bedrock,craigcook/bedrock,pascalchevrel/bedrock,craigcook/bedrock,MichaelKohler/bedrock,craigcook/bedrock,sylvestre/bedrock,ericawright/bedrock,hoosteeno/bedrock,pascalchevrel/bedrock,sylvestre/bedrock,alexgibson/bedrock,kyoshino/bedrock,alexgibson/bedrock,flodolo/bedrock,mozilla/bedrock,flodolo/bedrock,flodolo/bedrock,sgarrity/bedrock,mozilla/bedrock,MichaelKohler/bedrock,MichaelKohler/bedrock,MichaelKohler/bedrock,hoosteeno/bedrock,ericawright/bedrock,kyoshino/bedrock,kyoshino/bedrock,mozilla/bedrock,mozilla/bedrock,craigcook/bedrock,ericawright/bedrock,alexgibson/bedrock,alexgibson/bedrock,sylvestre/bedrock,sgarrity/bedrock,sylvestre/bedrock