Create tag and release for latest dcrstakepool

On GitHub [Releases](https://github.com/decred/dcrstakepool/releases) page, latest dcrstakepool release is v1.2.0 from 2019, and the latest tag is v1.5.0 from Jan 2020. This may lead to confusion like [this](https://github.com/decred/dcrwebapi/issues/138#issuecomment-841102467) for operators still willing to run dcrstakepool.