First iteration of schema checker page

What does this MR do and why?

This adds a schema checker diagnostic in addition to the collation checker diagnostic in /admin/database_diagnostics.

This is the frontend part. The backend has been split off to !200156 (merged).

References

#467544.

Screenshots or screen recordings

How to set up and validate locally

1. Checkout this branch.
2. Navigate to /admin/database_diagnostics.
3. Click on Run Schema Check.
4. You should see no issues.
5. If you want to create an issue, you can run gdk psql and drop an index:

DROP INDEX index_abuse_events_on_user_id;

6. Now click on Run Schema Check again.
7. You should see now a missing index.
8. Add back the index in gdk psql:

CREATE INDEX index_abuse_events_on_user_id ON abuse_events USING btree (user_id);

MR acceptance checklist

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