The query shouldn't be too expensive: it'll use an index to narrow
down the resultset for 1 user. After that, it'll be sorted based on
a grouped by value, but that should fit in memory: it'll never be
on more than 2000 entries, which is the max amount of notifications
per user.
Change-Id: I271ea7f7a6e010284739bfce02c4ec8a077148fc
