Make the "Host a new game" buttons primary on each page

Make the "Host a new game" buttons primary on each page

Displaying it first, (before the form for joining an existing game).

Also drop the submit button from the join form, (relying on the user
simply entering a game ID instead).

This makes the primary flow smoother: Choose a game, then click "Host"
without the user being confronted with a form and having to decide
which of two buttons to click.