Defaults to uploading to _internal which is wrong

[simonw]

As-of Datasette 0.54 a new _internal database is available to the root user - if you are signed in as root you get this, which is bad because _internal is an in-memory database not designed to handle uploaded content:

[simonw]

Bug is here:

datasette-upload-csvs/datasette_upload_csvs/__init__.py

Lines 45 to 46 in 245a9b7

	# For the moment just use the first database that's not immutable
	db = [db for db in datasette.databases.values() if db.is_mutable][0]