Avoid creation of branches in this workshop-template repo (and in styles)

[twitwi]

When we use the github importer to copy a repo (as in [1]), it copies all branches too.
We should thus avoid branches in this repository, as they would propagate to all new workshops (like [2]).

@rgaiacs @lexnederbragt

[1] https://github.com/swcarpentry/workshop-template#creating-a-repository
[2] https://github.com/twitwi/TEST-workshop-template3/branches

[rgaiacs]

@twitwi Thanks for the issue. I will check the branches and remove them if we already merged them.

@k8hertweck If we decide to solve #338 using branches, what I think was the decision so far, we will end up with a reasonable number of branches that will be duplicated on all the workshops pages.