Add code owner and enhance approval process

[xgreenx]

No description provided.

[netrome]

Discussed this in our retro. I'll have a look at it.

Definition of done

We have a structure for defining code owners, and enforce at least one approval from a code owner for touched parts of the code. Initially @xgreenx, @Dentosal and @MitchTurner will be code owners for the entire repo, but the structure should support migrating to more code owners of more specific pieces of the code.