[openstreetmap/openstreetmap-website] Give the changesets_count in changeset metadata (#2224)

[mmd]

Now with 100k table entries, explain result look a bit more like announced before:

```
explain verbose select count(user_id) from changesets where user_id = 1220430 and created_at < '2019-04-08 16:56:26.117157';
                                                                  QUERY PLAN                                                                  
----------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=4.44..4.45 rows=1 width=8)
   Output: count(user_id)
   ->  Index Only Scan using changesets_user_id_created_at_idx on public.changesets  (cost=0.42..4.44 rows=1 width=8)
         Output: user_id, created_at
         Index Cond: ((changesets.user_id = 1220430) AND (changesets.created_at < '2019-04-08 16:56:26.117157'::timestamp without time zone))

```

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