Fixed #17627 -- Renamed util.py files to utils.py

Thanks PaulM for the suggestion and Luke Granger-Brown and
Wiktor Kołodziej for the initial patch.
Added backwards compatability shims for util modules.

refs #17627
Fixed test failures introduced in previous commit.
Reworded a paragraph in the logging docs.
Fixed #4278 -- The `TEMPLATE_DIRS` setting can be overridden by
loader.get_template() and loader.select_template().
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