Why do we need two config sources part 2 electric boogaloo.

Continuation of #276 (closed) and discussion over IRC. New issue because turbodevs couldn't wait for me to actually comment on that issue.

So config.exs i needed because in currently proposed binary package FE is bundled in in such a way configuration for it is awkward.

config.exs/pleroma.conf + config.json if you set some flag in config.exs

Pros:

"easy" configuration in binary package (in quotes because it doesn't seem all that easy)

Cons:

one configuration for ALL the FEs, impossible to have separate configs unless you switch to config.json
to apply new config you need to restart entire server
need to sync PleromaFE settings to PleromaBE
FE devs have little control over what is synced to BE as we recently saw
FE devs don't really know how to configure FE in binary-package enviroment (logoMask vs logo_mask, where config file is located) therefore tough to help users struggling with new changes (i wrongly directed people to config.json initially)
Having a switch to disable config.exs will lead to FE devs to recommend people to toggling that off and using config.json instead

Pros:

easy configuration of FE itself
can have separate configs for different FEs (possible to run stable FE + unstable FE for example)
FE devs are happy

Cons: