Add indexes on user+id to changeset and diary comments

[tomhughes]

This adds indexes on user+id to diary and changeset comments to make comment views more efficient.

Once this is merged we can switch the diary_comments association on users to sort by id instead of creation date and then drop the creation date indexes.

[AntonKhorev]

Creation date indexes might still be useful for combining pagination with date pickers #5255 (comment)

[tomhughes]

Well then you'd ideally want an index on author+date+id though we have to think about how much space we want o use on indexes at some point - is here really a need for filtering by date? How often do you know that you need to look at comments from a particular date?

[tomhughes]

Those indexes are all pretty tiny currently so I guess keeping them doesn't hurt.

[AntonKhorev]

filtering by date

Not filtering really, just get the starting id by date and then use it as before or after.

[AntonKhorev]

an index on author+date+id

+id is probably not necessary. If a user managed to create multiple objects during the same microsecond we can afford picking a wrong one. Or select min/max(id) where created_at is between target_date and target_date+/-epsilon.