Support creating a table migration generator

Sometimes you want to create a table without an associated model and test, which is also not a join table. With this commit, you can now do that.

I've typed these commands enough times over the years and ended up with empty migrations that I finally decided to add them to Rails and see if anyone else considered it useful.

Example:

    rails g migration create_posts title:string
or
    
    rails g migration CreatePosts title:string

This commit also moves the template the model generator uses for the migration to the migration templates folder, as it seems a more sensible place for it now that it is shared code.