Add a config option to override the prefix in SD chat messages; use config file for prefix configuration

[CoconutMacaroon]

This PR adds a config option that provides a setting to override the entire prefix for messages posted by SD in chat. This would allow one to add any link as a prefix (not just to the GitHub repo), but a user could also set the prefix to something else as well.

It also changes the repo/bot name prefix options to use the config file instead of env variables.

[CoconutMacaroon]

I changed the one to a config option, I’ll change the other ones there to config options as well later today

[stale[bot]]

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[CoconutMacaroon]

I've converted the other two options to the config file. I also tested this change here, and it seems to work as expected.

[makyen]

I'm sorry for the delay in merging this. I really need to get better at keeping track of these and actually dealing with them.