Don't allow writable API to edit the _memory database

[simonw]

It shows up on https://latest.datasette.io/-/api (once you are signed in as root) - but there's no point in creating tables in it because they likely won't persist from one request to the next, as it's not a shared named database.

[simonw]

The problem might actually be here:

datasette/datasette/app.py

Lines 280 to 281 in 9f5321f

	if memory or crossdb or not self.files:
	self.add_database(Database(self, is_memory=True), name="_memory")

is_mutable defaults to True, so this line should probably be:

     self.add_database(Database(self, is_mutable=False, is_memory=True), name="_memory")

[simonw]

Yeah it looks like I introduced this bug here:

fb7e70d