DEV: remove exec_sql and replace with mini_sql

Introduce new patterns for direct sql that are safe and fast.

MiniSql is not prone to memory bloat that can happen with direct PG usage.
It also has an extremely fast materializer and very a convenient API

- DB.exec(sql, *params) => runs sql returns row count
- DB.query(sql, *params) => runs sql returns usable objects (not a hash)
- DB.query_hash(sql, *params) => runs sql returns an array of hashes
- DB.query_single(sql, *params) => runs sql and returns a flat one dimensional array
- DB.build(sql) => returns a sql builder

See more at: https://github.com/discourse/mini_sql

app/models/user_visit.rb

@@ -11,7 +11,7 @@ class UserVisit < ActiveRecord::Base
   end
 
   def self.count_by_active_users(start_date, end_date)
-    sql = <<SQL
+    sql = <<~SQL
       WITH dau AS (
         SELECT date_trunc('day', user_visits.visited_at)::DATE AS date,
                count(distinct user_visits.user_id) AS dau
@@ -27,9 +27,9 @@ class UserVisit < ActiveRecord::Base
           WHERE user_visits.visited_at::DATE BETWEEN dau.date - 29 AND dau.date
         ) AS mau
       FROM dau
-SQL
+    SQL
 
-    UserVisit.exec_sql(sql, start_date: start_date, end_date: end_date).to_a
+    DB.query_hash(sql, start_date: start_date, end_date: end_date)
   end
 
   # A count of visits in a date range by day
@@ -42,16 +42,16 @@ SQL
   end
 
   def self.ensure_consistency!
-    exec_sql <<SQL
-    UPDATE user_stats u set days_visited =
-    (
-      SELECT COUNT(*) FROM user_visits v WHERE v.user_id = u.user_id
-    )
-    WHERE days_visited <>
-    (
-      SELECT COUNT(*) FROM user_visits v WHERE v.user_id = u.user_id
-    )
-SQL
+    DB.exec <<~SQL
+      UPDATE user_stats u set days_visited =
+      (
+        SELECT COUNT(*) FROM user_visits v WHERE v.user_id = u.user_id
+      )
+      WHERE days_visited <>
+      (
+        SELECT COUNT(*) FROM user_visits v WHERE v.user_id = u.user_id
+      )
+    SQL
   end
 end