Fixed #23840 -- Fixed makemessages find_files method

Changed the handling of extensions to be used for gettext. Now
obeying the --extension argument. find_files now only find the
given or default extensions and puts only these in the
TranslatableFiles. As a result there are no more confusing messages
for filetypes (extension) not handled by makemessages.
Simplified handle_extensions management utility

makemessages now doesn't need any special ignoring logic, after
commit bb4a92d784.
Improved readability of negative condition in makemessages

Thanks Berker Peksag for the suggestion.
Added assertion error message for test_make_line

Refs #23842. Thanks Thomas Chaumeny for the suggestion.
Fixed #20435 -- Reordered the custom template tags docs.

Introduced the various shortcuts before explaining the more complex
parser/render functionality.

Also removed non-decorator syntax: it's been years since Django
supported a Python version without decorators.
Moved bilateral transform fetching to models.lookups
Fixed #23855 -- Removed unnecessary all() in tutorial 3.
Fixed #18714 -- Added 'fuzzy' compilemessages option