formats: Treat converted PO files nearly as bilingual

[nijel]

Proposed changes

Converting location to context is wrong because it is not guaranteed to
be unique. Actually, all conversion engines merge same strings into
single unit.

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

TODO:

• Add migrations for existing units (adjust id_hash, move context to location)

Other information

[codecov]

Codecov Report

Merging #7585 (c0e9c3c) into main (019c333) will increase coverage by 0.44%.
The diff coverage is n/a.

❗ Current head c0e9c3c differs from pull request most recent head 0caeaf3. Consider uploading reports for the commit 0caeaf3 to get more accurate results

@@            Coverage Diff             @@
##             main    #7585      +/-   ##
==========================================
+ Coverage   90.58%   91.02%   +0.44%     
==========================================
  Files         658      658              
  Lines       48438    49463    +1025     
  Branches     7104     7618     +514     
==========================================
+ Hits        43876    45023    +1147     
+ Misses       3166     3072      -94     
+ Partials     1396     1368      -28     

Impacted Files	Coverage Δ
weblate/api/tests.py	100.00% <0.00%> (ø)
weblate/lang/forms.py	100.00% <0.00%> (ø)
weblate/checks/data.py	100.00% <0.00%> (ø)
weblate/legal/tests.py	100.00% <0.00%> (ø)
weblate/checks/tasks.py	100.00% <0.00%> (ø)
weblate/utils/models.py	100.00% <0.00%> (ø)
weblate/billing/tests.py	100.00% <0.00%> (ø)
weblate/wladmin/tests.py	100.00% <0.00%> (ø)
weblate/api/throttling.py	100.00% <0.00%> (ø)
weblate/utils/decorators.py	100.00% <0.00%> (ø)
... and 171 more