Add index + unique constraint on issue_id and comment_id

[SimonLab]

The current schemas for issue and comment don't have any unique constraint and index on issue_id and comment_id yet.
This can be useful to make sure the data inserted are correct and to avoid duplicates.
A duplicate values can appear for example if the app is uninstalled and reinstalled again. As we won't automatically delete data in Postgres (see #87) this means that on the install we will save again all the issues and comments (see #65)
After adding these indexes and constraints we will need to update the Repo.insertfunction on new install toRepo.insert_or_update`:

github-backup/lib/app_web/controllers/event_type_handlers.ex

Line 69 in 2960a82

issue = Repo.insert!(changeset)

see https://hexdocs.pm/ecto/Ecto.Repo.html#c:insert_or_update/2

[nelsonic]

This will certainly speed up queries. 🐎