Resolve "Hydra upgrade blocked by missing mariadb feature"

Closes #1483

Warning

This is not meant to be merged in a single stackspin release.

Step one: stackspin 2.15

• Add mysql as new stackspin database abee0b0b

Step two: stackspin 2.16

• Migration to copy stackspin database data 6876dd68
• Switch to using mysql stackspin database 191b7b6d

Step three: stackspin 2.17

• Remove old mariadb stackspin database 0c025474