MOB-2213: Add information about poi count to map CRUD API

Thanks for your feedback guys.

@nandy-andy From what I see adding join on "poi" does not change much, because it is using index, and the heavy part in that query seem to be the sort:


```
explain select `map`.`id`, `map`.`title`, `tile_set`.`image`, `map`.`updated_on`, `tile_set`.`status`, `tile_set`.`id` as `tile_set_id` from `map` inner join `tile_set` on `tile_set`.`id` = `map`.`tile_set_id` where `deleted` = false and `status` = 0 order by `map`.`created_on` desc;
+----+-------------+----------+------+----------------------------------+-----------------+---------+------------------------------+------+---------------------------------+
| id | select_type | table    | type | possible_keys                    | key             | key_len | ref                          | rows | Extra                           |
+----+-------------+----------+------+----------------------------------+-----------------+---------+------------------------------+------+---------------------------------+
|  1 | SIMPLE      | tile_set | ref  | PRIMARY,tile_set,tile_set_status | tile_set_status | 1       | const                        |   90 | Using temporary; Using filesort |
|  1 | SIMPLE      | map      | ref  | tile_set_id                      | tile_set_id     | 4       | interactive_maps.tile_set.id |    1 | Using where                     |
+----+-------------+----------+------+----------------------------------+-----------------+---------+------------------------------+------+---------------------------------+
```

@RafLeszczynski It seems that currently there is no API that returns data I need. /api/v1/poi returns list of all pois (global) without information about associated wikia id nor with support for filtering by wikia id in the param. Do you know about some other API I could user? If not then I could add city_id filtering and map_id to the output data (so I can do per map count on the client side).