Merge branch 'fix_issue_15259' into 'master'

Fix bug with SQL syntax error during backup restoration

## What does this MR do?
It properly escapes the table name in the `DROP TABLE IF EXISTS ...` statement used during restore of a backup.

## Are there points in the code the reviewer needs to double check?
Please check compatibility with PostgreSQL

## Why was this MR needed?
Fix an issue with backups can not be restored if MySQL is used as database for GitLab.

## What are the relevant issue numbers?
#15259 

## Screenshots (if relevant)
-

See merge request !4318

lib/tasks/gitlab/db.rake

@@ -34,7 +34,7 @@ namespace :gitlab do
       # PG: http://www.postgresql.org/docs/current/static/ddl-depend.html
       # MySQL: http://dev.mysql.com/doc/refman/5.7/en/drop-table.html
       # Add `IF EXISTS` because cascade could have already deleted a table.
-      tables.each { |t| connection.execute("DROP TABLE IF EXISTS #{t} CASCADE") }
+      tables.each { |t| connection.execute("DROP TABLE IF EXISTS `#{t}` CASCADE") }
     end
 
     desc 'Configures the database by running migrate, or by loading the schema and seeding if needed'