Fix papercuts

[pietroalbini]

I'm making an unrelated change to the RFD API, and while setting up my local environment I encountered a few papercuts that would be nice to have fixed:

• log_format was present in rfd-api but not in rfd-processor, forcing the use of the JSON logs there. I added the option to it, making sure to maintain backward compatibility with old configuration files.

• While making the previous change, I noticed that rfd-api contained a lot of code to manually deserialize the log format. I switched that enum to the serde derive.

• Running the migrations, schema.rs was updated with a whitespace diff and a bunch of tables from v-api. I modified diesel.toml to ensure there is zero diff when running migrations.

• While doing so, I noticed the migrations directory field didn't exist, so I removed it to avoid future confusion.

• Updated the setup instructions to mention the correct way to run database migrations, as discussed on Matrix.

[augustuswm]

Thanks so much for doing this. Greatly appreciated and needed!