Add support for separate Horizon deployment

[holysoles]

I found we need to make sure a queue worker (Horizon) starts, otherwise certain things dont work (i.e. deleting posts). While I opened this PR for the docker image, as Matt notes, it'd probably be better for scalability to run it in a separate deployment.

Changes in MR:

• in current state, set the new image ENABLE_HORIZON env var to true
• if .Values.pixelfed.horizon.separate_deployment is true, create deployment_backend.yaml which deploys the same app image, but with an overridden command/args to run Horizon explicitly (related docs suggesting this for compose). I've also removed some env vars that I don't expect to need for that process. More cleanup may be possible.
• I had to adjust labels to ensure only the frontend/web pod gets selected by the service. Certainly open to edit suggestions there

We could make this the default, but for complexities sake, might not be worth it

[jessebot]

OK, this seems good to me. Let's keep it as optional for now, at least until I can properly test it out :) Thank you as always for your contributions!

[jessebot]

Side comment, but for some reason, the helm docs action I run on this repo never works on your branches. It works when I open a PR directly on this repo though. The whole point of it is supposed to be that it updates your branch for you, so that you don't have to remember to run helm-docs locally. Maybe it's a permissions thing? We can worry about that later though. For now, I just committed the doc updates directly myself. :)

[holysoles]

Side comment, but for some reason, the helm docs action I run on this repo never works on your branches. It works when I open a PR directly on this repo though. The whole point of it is supposed to be that it updates your branch for you, so that you don't have to remember to run helm-docs locally. Maybe it's a permissions thing? We can worry about that later though. For now, I just committed the doc updates directly myself. :)

I've noticed it doesnt work for me as well.. I'll look later to see if i can notice anything about why