Guidelines for history extraction from bc

@gvwilson, @wking @r-gaia-cs, continuing discussion here.

It would be good to have a central reference (blog post?) for the
process of lesson history extraction. The current instructions for
this are a bit diffuse.

I think there needs to be clarification on the following (as I type, no order):

1) Where progress is tracked (currently labeled issues on bc)

2) Where new PRs on lesson material should be made.

3) Can these PRs be merged or are they pending conversion to the new
lesson template? If merging then how to manage overview on personal
repos?

4) Should we do this entirely in personal repos or make repos in swcarpentry?

5) Is bc frozen?


Additionally:

6) If (3) is true should an extra step in the extraction be to put
everything in a pages/ directory, following the new lesson template?
Otherwise we have to filter-branch again when merging PRs.
I agree - definitely freeze bc. What I'm concerned about is moving the *issues themselves* from interim to final. 

e.g. Say `user` has created issue `#1` on `interim` repo owned by `maintainer` and then the content of `interim` is moved to another repo, `final`, without `#1` being dealt with - who's responsibility is it to re-create `#1` on `final`? 

Can we afford to just drop issues that aren't re-created by `user` or do we need to make an effort to deal with all issues before transitioning to `final`?