Fixed #18336 -- Doubled request_queue_size.

This reduces random failures to load static files with Google Chrome.
Pointed to the docs version switcher in tutorial 1.

Refs #20324. Thanks dwisehart for the suggestion.
Fixed #18986 -- Improved error message for missing files

in CachedStaticFilesStorage. Thanks zyegfryed for his work on the patch.
Tested that get_or_create raises IntegrityError.

It used to raise "DatabaseError: no such savepoint" with the old
transaction management. Closes #15117.
Corrected "it's" to "its" when used possessively.

Fixed #20327.