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

script/import_scripts/base.rb

@@ -626,7 +626,7 @@ class ImportScripts::Base
   def update_topic_status
     puts "", "Updating topic status"
 
-    Topic.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE topics AS t
       SET closed = TRUE
       WHERE EXISTS(
@@ -636,7 +636,7 @@ class ImportScripts::Base
       )
     SQL
 
-    Topic.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       UPDATE topics AS t
       SET archived = TRUE
       WHERE EXISTS(
@@ -646,7 +646,7 @@ class ImportScripts::Base
       )
     SQL
 
-    TopicCustomField.exec_sql(<<~SQL)
+    DB.exec(<<~SQL)
       DELETE FROM topic_custom_fields
       WHERE name IN ('import_closed', 'import_archived')
     SQL
@@ -654,7 +654,7 @@ class ImportScripts::Base
 
   def update_bumped_at
     puts "", "Updating bumped_at on topics"
-    Post.exec_sql("update topics t set bumped_at = COALESCE((select max(created_at) from posts where topic_id = t.id and post_type = #{Post.types[:regular]}), bumped_at)")
+    DB.exec("update topics t set bumped_at = COALESCE((select max(created_at) from posts where topic_id = t.id and post_type = #{Post.types[:regular]}), bumped_at)")
   end
 
   def update_last_posted_at
@@ -674,7 +674,7 @@ class ImportScripts::Base
         AND users.last_posted_at <> lpa.last_posted_at
     SQL
 
-    User.exec_sql(sql)
+    DB.exec(sql)
   end
 
   def update_user_stats
@@ -707,7 +707,7 @@ class ImportScripts::Base
         AND user_stats.first_post_created_at <> sub.first_post_created_at
     SQL
 
-    User.exec_sql(sql)
+    DB.exec(sql)
 
     puts "", "Updating user post_count..."
 
@@ -725,7 +725,7 @@ class ImportScripts::Base
         AND user_stats.post_count <> sub.post_count
     SQL
 
-    User.exec_sql(sql)
+    DB.exec(sql)
 
     puts "", "Updating user topic_count..."
 
@@ -743,15 +743,15 @@ class ImportScripts::Base
         AND user_stats.topic_count <> sub.topic_count
     SQL
 
-    User.exec_sql(sql)
+    DB.exec(sql)
   end
 
   # scripts that are able to import last_seen_at from the source data should override this method
   def update_last_seen_at
     puts "", "Updating last seen at on users"
 
-    User.exec_sql("UPDATE users SET last_seen_at = created_at WHERE last_seen_at IS NULL")
-    User.exec_sql("UPDATE users SET last_seen_at = last_posted_at WHERE last_posted_at IS NOT NULL")
+    DB.exec("UPDATE users SET last_seen_at = created_at WHERE last_seen_at IS NULL")
+    DB.exec("UPDATE users SET last_seen_at = last_posted_at WHERE last_posted_at IS NOT NULL")
   end
 
   def update_feature_topic_users