Reorganise the main file saving templates

We’re introducing a mechanism that allows us to decide which tiddlers
get saved. Using `$:/core/save/all` gives all tiddlers, as before.
Using `$:/core/save/empty` gives an empty wiki. Saving an empty wiki
from tw5.com isn’t the same as the empty wiki cooked from
editions/empty because the two editions have different plugins and
other system tiddlers.