Implement simple migration view.

The migration view can be invoked like this:

http://localhost/github-book/#repo/oerpub/test-book/migrate/[taskname]

At the moment there is only one task named "tables", so you can call:

http://localhost/github-book/#repo/oerpub/test-book/migrate/tables

When the migration is done, you should see the asterisk in the picker/toc that indicates which modules were modified. You can open them and inspect the result before saving.

This is more of a proof of concept for now.

I am uncertain whether this code needs to be github specific, or also available for atc. In its present form, some parts are available to atc and can be used there.

The migration view needs a bit of styling love.

I think it needs to list the available tasks when the user navigates to the migration view without specifying one. It is already coded in a way that allows the task to be optional, I just don't handle that right.

This PR is more for testing and comment.