Split db-init cli in 2 (init and create) and move them under 'swh db'

The idea it to have the 'swh db create' command to create the database
and prepare it by executing SQL -superuser- files, i.e. SQL files which
execution requires superuser permissions -- typically install languages
or extensions.

Then the 'swh db init' execute the initialization SQL files that do not
require superuser permissions.

Also get rid of the previous (unused) 'swh db init' command.