Fix: add index on comment.object_id column

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

When lots of comments are stored in database, GET api/object/<id>/comment endpoint timeouts due to lack of index on comment.object_id.

What is the new behaviour?

Added missing index which improved query time.