Feat: add course instance to completion

[mipyykko]

As completions are for some courses handled by a common handler course or parent, we lose the information on what course the original was, apart from the language mapping which is not tied to the course and may actually be erroneous since it comes from user course settings. Having the original instance also helps in querying certificates API, since the certificates are connected to the language version instance there.

Also pass the instance down to completion model for the queries that come from through instances that still have the information on the original course.

[codecov]

Codecov Report

Merging #1032 (b0b627c) into master (597be33) will decrease coverage by 0.05%.
The diff coverage is 56.81%.

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
- Coverage   67.55%   67.50%   -0.06%     
==========================================
  Files         109      109              
  Lines        4222     4240      +18     
  Branches      916      922       +6     
==========================================
+ Hits         2852     2862      +10     
- Misses       1267     1274       +7     
- Partials      103      104       +1     

Flag	Coverage Δ
backend	67.50% <56.81%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
backend/graphql/Completion/mutations.ts	57.14% <0.00%> (-0.69%)	⬇️
backend/util/server-functions.ts	84.12% <ø> (ø)
backend/graphql/Completion/queries.ts	26.53% <20.00%> (ø)
backend/graphql/Completion/model.ts	40.00% <30.00%> (-0.75%)	⬇️
backend/graphql/User/model.ts	41.12% <33.33%> (-0.39%)	⬇️
backend/graphql/UserCourseSummary.ts	55.00% <33.33%> (+1.15%)	⬆️
backend/api/routes/progress.ts	32.12% <50.00%> (ø)
backend/api/routes/completions.ts	58.75% <66.66%> (+0.36%)	⬆️
backend/util/db-functions.ts	58.40% <83.33%> (+1.39%)	⬆️
...n/kafkaConsumer/common/getUserWithRaceCondition.ts	100.00% <100.00%> (ø)
... and 3 more

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