Switch to 2-branch system for styles repository

[fmichonneau]

@rgaiacs proposed a 2-branch system for the styles repository that would allow us to not have to rely on the the boilerplate files and the python script to initialize new lessons. carpentries/styles#287

[fmichonneau]

@rgaiacs Will you be able to work on implementing this?

[rgaiacs]

Will you be able to work on implementing this?

I will add it to my list but probably only at the end of July or begin of August.