Introduce the concept of a "comp"

A comp is a composition, which for now is a recipe which contains
at least one tiddler named "app". Loading /comp/foobar looks for
app in the foobar recipe and then displays it (as a single tiddler).

This allows for tanks to be paired with "apps".

This is highly expermental at this point: there is no UI for
creating a comp or recipe.
Include an empty.html of tiddlywiki classic as an "app"

This doesn't work yet, as the tiddlywiki does not know to load
content from the server. However if you have a recipe named foo of

  /bags/tiddlywiki/tiddlers
  /bags/sometank/tiddlers

When going to /comps/foo you get a tiddlywiki.

We need to add TiddlyWebAdaptor and friends, and boostrapping code
to load the initial tiddlers from the right place.

This will likely seem a bit slow, but ought to be no slower than
existing TiddlyWiki handling _and_ it clarifies the slowness as
in TiddlyWiki and give clear room for where improvement needs to
happen.