Fix filters that break i18n

[int-y1]

These issues were found with git grep "_(.*=.*|"

• Change as_countdown to a registry.function. Fix issues with |timedelta. Fix pp messages.
• Test that all affected pages still work properly.
• Test that makemessages can find the i18n strings:

python manage.py makemessages -l en -e py,html,txt
git diff upstream/update-i18n -- locale/en/LC_MESSAGES/django.po

[codecov-commenter]

Codecov Report

Merging #1939 (9782861) into master (966b37d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1939   +/-   ##
=======================================
  Coverage   46.46%   46.46%           
=======================================
  Files         236      236           
  Lines       13114    13114           
=======================================
  Hits         6093     6093           
  Misses       7021     7021           

Impacted Files	Coverage Δ
judge/jinja2/timedelta.py	58.82% <100.00%> (ø)

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[Ninjaclasher]

LGTM

[Xyene]

LGTM, thanks!