[BD-21] Migrate waffle flag classes to edx-toggles
[BD-21] Migrate waffle flag classes to edx-toggles

The same API is preserved, internally, to avoid many changes across the
edx-platform codebase (for now).

stvstnfrd/edx-platform,EDUlib/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,stvstnfrd/edx-platform,edx/edx-platform,edx/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,eduNEXT/edunext-platform,arbrandes/edx-platform,EDUlib/edx-platform,eduNEXT/edunext-platform,edx/edx-platform,stvstnfrd/edx-platform,arbrandes/edx-platform,stvstnfrd/edx-platform,EDUlib/edx-platform,EDUlib/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,eduNEXT/edunext-platform,angelapper/edx-platform,angelapper/edx-platform,angelapper/edx-platform,eduNEXT/edunext-platform,edx/edx-platform