Fixed #21196 -- Removed index on test field causing MySQL specific warning.
Fixed #23684 -- Used correct package name for Pillow in Error hint.
Fixed a typo in urlencode documentation.
Merge pull request #3390 from wrwrwr/fix/urlencode-ref-typo

Fixed a typo in urlencode documentation.
Prevented syncdb from calling system checks multiple times. Refs #23650.
Renamed skip_validation into skip_checks as the former is now deprecated.
Made the ping_google command override BaseCommand.handle() rather than execute().

Overriding execute() bypasses the system check framework among other things.
Moved a test case that caused deprecation warnings.

`cache.tests.TestEtagWithAdmin` loaded views from the `admin_views` test
package. This is problematic because when the `cache` test package is
run in isolation, `admin_views` isn't in INSTALLED_APPS, and therefore
loading its models isn't allowed since the app loading refactor.