refactor: use migrations that work on PostgreSQL also on H2 and MySQL to reduce duplication

Relate to #1034

Author: php-coder

src/main/resources/liquibase/version/0.3/2014-09-28--collection_slug.xml

@@ -14,25 +14,12 @@
 
 	</changeSet>
 
-	<changeSet id="update-collections-slug" author="php-coder" context="test-data,prod-data" dbms="mysql,h2">
+	<changeSet id="update-collections-slug" author="php-coder" context="test-data,prod-data">
+		<validCheckSum>8:f11bf0ed0e440867152a117a7447b8c5</validCheckSum>
 		<comment>Sets value of slug field to transformed collection's owner's name</comment>
 
 		<!-- Unfortunately, H2 doesn't support UPDATE with JOIN -->
-		<sql>
-			UPDATE collections c
-			SET c.slug = (
-				SELECT LOWER(REPLACE(u.login, ' ', '-'))
-				FROM users u
-				WHERE u.id = c.user_id
-			);
-		</sql>
-		
-	</changeSet>
-	
-	<changeSet id="update-collections-slug" author="php-coder" context="test-data,prod-data" dbms="postgresql">
-		<comment>Sets value of slug field to transformed collection's owner's name</comment>
-		
-		<!-- In PostgreSQL it is illegal to prefix columns with table alias in the SET clause -->
+		<!-- NOTE: In PostgreSQL it is illegal to prefix columns with table alias in a SET clause -->
 		<sql>
 			UPDATE collections c
 			SET slug = (

src/main/resources/liquibase/version/0.4/2018-06-09--add_number_of_stamps_to_collections_series.xml

@@ -13,31 +13,12 @@
 
 	</changeSet>
 
-	<changeSet id="fill-collections_series-number_of_stamps-field" author="php-coder" context="prod-data" dbms="mysql,h2">
+	<changeSet id="fill-collections_series-number_of_stamps-field" author="php-coder" context="prod-data">
+		<validCheckSum>8:89cf638686e11255fd45dc41c1250c9d</validCheckSum>
 		<comment>Sets value of number_of_stamps field to number of stamps in a series</comment>
 
-		<sql>
-			UPDATE collections_series cs
-			SET cs.number_of_stamps = (
-				SELECT s.quantity
-				FROM series s
-				WHERE s.id = cs.series_id
-			)
-		</sql>
-		
-		<rollback>
-			<sql>
-				UPDATE collections_series
-				SET number_of_stamps = NULL
-			</sql>
-		</rollback>
-		
-	</changeSet>
-	
-	<changeSet id="fill-collections_series-number_of_stamps-field" author="php-coder" context="prod-data" dbms="postgresql">
-		<comment>Sets value of number_of_stamps field to number of stamps in a series</comment>
-		
-		<!-- In PostgreSQL it is illegal to prefix columns with table alias in the SET clause -->
+		<!-- Unfortunately, H2 doesn't support UPDATE with JOIN -->
+		<!-- NOTE: In PostgreSQL it is illegal to prefix columns with table alias in a SET clause -->
 		<sql>
 			UPDATE collections_series cs
 			SET number_of_stamps = (