Make it easy to set up live servers, probably using Kamal

[kimadactyl]

User story

As a developer, I want to be able to easily set up and tear down test servers so I can have my own testing environment and we are not so shackled to GFSC's staging server.

Dokku is what's currently described in our INSTALL.md and this felt state of the art in 2017 when we were getting going but there's now probably better things, most notably Kamal which is created by the Rails team.

@lenikadali shared some thoughts here: https://gist.github.com/lenikadali/d76147db5fc5e9e4c037d84638cd0930?permalink_comment_id=5240007#gistcomment-5240007

Acceptance criteria

• You can run one (or a small number) of commands to get a PlaceCal server up and running
• You can run one (or a small number) of commands to strike down the PlaceCal server
• Optional: PlaceCal main branch is reconfigured to use this new method
• Optional: PlaceCal production branch is reconfigured to use this new method

Implementation notes & questions

Implementation plan

To be written by the developer