Add translator_last_activity and reviewer_last_activity time columns

[jennifer-lu]

Notion ticket link

Add translator_last_activity and reviewer_last_activity time columns to StoryTranslation model

Implementation description

• Added translator_last_activity and reviewer_last_activity time columns to database

Steps to test

1. Upgrade database docker exec -it planet-read_py-backend_1 /bin/bash -c "flask db upgrade"
2. Check that new columns translator_last_activity and reviewer_last_activity have been added to the tables story_translations and story_translations_all
   a. docker exec -it planet-read_db_1 /bin/bash
   b. mysql -u root -proot
   c. use planet-read
   d. select * from story_translations;
   e. select * from story_translations_all;

What should reviewers focus on?

• Database migration is correct
• translator_last_activity and reviewer_last_activity time columns appear in database

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• For backend changes, I have run the appropriate linters: docker exec -it planet-read_py-backend_1 /bin/bash -c "black . && isort --profile black ." and I have generated new migrations: flask db migrate -m "<your message>"
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[gaoxk]

nice!