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.
Reordered condition to avoid calling allow_migrate() if unneeded.
Avoided unneeded calls to state.render() in migrations.
Fixed #23859 -- Fixed a migration crash when a field is renamed that is part of an index_together
Merge pull request #3563 from MarkusH/ticket23859

Fixed #23859 -- Fixed a migration crash when a field is renamed that is part of an index_together
Removed superfluous newline.
Normalized check that ALLOWED_INCLUDE_ROOTS is a tuple.
Adjusted tests for previous commit.

The test was testing a use case that doesn't happen in real world
projects: developers don't assign settings at run time (and Django
explicitly doesn't support it).
Fix link target markup

This markup for a code block is redundant with the code-block directive below,
and blocks the following line from working as link target.
Fix malformed note directives.