Create Training

Put any number of trainees on a trainer's blocks for a dated window, new hire or retraining -- CHANGES.md §2. Gated on people:write, the same grant /supervisor-training above already uses: both change what a published schedule is allowed to hold.

⛔RETRAINING DOES NOT PULL THE TRAINEE OFF THEIR OWN BLOCKS YET. CHANGES.md §2, verbatim: "Retraining does not yet pull the person off their usual blocks on the grid. Frederick to decide; build as the design shows." This route only ever writes workforce_trainings/ workforce_training_trainees (see training.py's own header) -- it never touches floor_blocks/roster_shifts, so nobody is removed from a shift they already hold by calling it.