Resurrecting the MySQL backend

MySQL remains a serious choice over Postgres in many domains. We should support it.
There exists a [`beam-mysql`](https://github.com/tathougies/beam-mysql), but it is unmaintained. `beam-mysql` is based on [`mysql`](https://hackage.haskell.org/package/mysql), which is also unmaintained.

We should:
* import [`beam-mysql`](https://github.com/tathougies/beam-mysql) in this repo;
* Switch over to use [`mysql-haskell`](https://github.com/winterland1989/mysql-haskell), which is maintained

I'm a little oversubscribed at the moment, but I'm happy to review pull requests!

Start by inspecting the existing database backend integrations in this repository and compare them with beam-mysql. Then review beam-mysql and mysql-haskell to determine the import and dependency changes needed. Done means the MySQL backend is included here and uses the maintained mysql-haskell package; the issue author is available to review pull requests.

Written by the indexing model from the issue text.