Allow external tools to inject backup agents

[luismedel]

I created a self contained Nullboard docker image (html app + backup agent) using a custom backup agent.

For the thing to work, I needed a way to inject into Nullboard the agent auth token. So I moved all the defaults for the mandatory backup agents to a set of constants, in order to use an external tool (sed) and modify it's values during the first container run.

I know this change is a bit ad-hoc for my needs, so feel free to discard it if you consider it doesn't fit.

Also I removed a rocgue curly brace that VSCode detected in the CSS :-)

[apankrat]

Hi Luis, I reworked your patch a bit to be more consistent with the rest of the code, but still allowing for easy grepping and sed'ing if needed. See commit db65363. Your script would need to search and patch lines containing "local agent" and "remote agent". I hope this is acceptable.

[luismedel]

Thank you @apankrat! I'll check it out.