RSS: Make a separate RSS for posts about MySQL.

[dbazhenov]

In our Community Blog, we accept articles from authors on various database-related topics. Some aggregators and community members only want to read articles related to MySQL. We need to figure out how to make separate RSS for different tags of our blog. Tag mysql-and-variants articles suitable for export to aggregators and directories.

Related task https://github.com/oursqlcommunity-org/planet/issues/68