Ability to add custom actions

I'm curious if the ability of adding user scripts that are available to run from the web frontend, under Actions, is of any interest, or something that's been discussed in the past and the maintainers are vehemently against.

## Why

It would be helpful to add our own scripts that we can run by just visiting the web app.

## What kind of scripts?

* maintenance script
* restarting a custom service
* anything you usually SSH in to do

## How would they get added?

I was thinking they could be added the same way steam entries are added. Users can just write YAML files.

## Concerns

Obviously, exposing scripts that run on the system could be very dangerous. But I like to think technology should treat people like adults, so it's on them.

No files or tests are named. Start by reviewing how Steam entries are represented and how the web frontend exposes Actions, then clarify the scope and safety requirements for user-supplied scripts; done should mean a concrete, maintainer-approved approach for running them from the web app.

Written by the indexing model from the issue text.