db: Make 'text' field in 'comments' table NOT NULL and handling data migration

[pkvach]

Checklist

• All new and existing tests are passing
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)
  

What changes does this Pull Request introduce?

This update introduces a schema migration to version 4 for the database, focusing on enhancing the 'comments' table. This ensures that the 'text' field in the 'comments' table will always have a value, which improves data consistency and integrity.

The key changes:

• Incrementing the database schema version from 3 to 4.
• Updating the "text" field in the "comments" table to be NOT NULL by setting empty strings for any NULL values.
• Creating a new table, "comments_new", with a NOT NULL constraint on the "text" field and then migrating data from the old "comments" table to this new table.
• Renaming the original "comments" table to "comments_backup_v3" for backup purposes and renaming "comments_new" to "comments" to finalize the migration.
• The migration process includes transaction handling to ensure data integrity, with a rollback in case of any errors during the migration.

Why is this necessary?

• NULL text from deleted comment results in error 500 #979
• isso: migrate: Handle deleted comments in Disqus migration #994

[ix5]

Nice effort, thank you.

I noticed that these lines should have been a migration as well:

isso/isso/db/comments.py

Lines 44 to 47 in 27bdc72

	try:
	self.db.execute(['ALTER TABLE comments ADD COLUMN notification INTEGER DEFAULT 0;'])
	except Exception:
	pass

Could you add them while you're at it?

[pkvach]

Nice effort, thank you.

I noticed that these lines should have been a migration as well:

isso/isso/db/comments.py

Lines 44 to 47 in 27bdc72

	try:
	self.db.execute(['ALTER TABLE comments ADD COLUMN notification INTEGER DEFAULT 0;'])
	except Exception:
	pass

Could you add them while you're at it?

Thanks for the review.
I have added the suggested changes. I will do squash commits once the code review is complete.

[ix5]

This is a massive improvement, thank you! A few suggestions inline, feel free to ignore if you feel they are too nitpicky.

[pkvach]

@ix5
Thank you for your suggestions. I've made the changes.

[ix5]

Squashed (pushed to your branch) and merged.

I have a few changes that I wrote and stashed locally when reviewing this, but that's for another day. This PR is a great improvement that allows for more confidence when releasing the next version.

[pkvach]

Great, thank you.