Added several indexes to feed_subscriptions table.

The new indexes are:

- feed_id (it speeds up joining with feeds table, which happens when looking for a user's feeds)
- user_id (it speeds up filtering by user_id, which happens when looking for all feeds subscribed by a user regardless of unread entries)
- user_id and unread_entries (it speeds up filtering by user_id and unread_entries > 0, which happens when looking for feeds subscribed by a user which hafe unread entries)
Added index on title column to feeds table.

It is useful when ordering feeds by the title column. The index means no reordering is necessary, because if the query plan uses the index records are returned in the desired order.