Create main branch, set as default, delete master

[stefano.menci]

I think it’s impossible to rename or delete master.

In some apps I have created my own branches with my own names and left master there, in one of the first commits.

I already mentioned this when the new git management system was deployed:

@stefano.menci

But I don’t see any other reason to call that branch master other than that’s what the other guys do outside of Anvil.