Mix tasks in an OTP release

Pleroma is almost ready from being OTP release ready. The remaining issue is the mix tasks. Mix is a dev tool, and it's not present in an OTP releases.

Instead, we can run mfa RPC calls into the running app node: we just have to change the existing tasks to not use any of the Mix functions at runtime (Mix.shell, …) while still respecting the Mix.Task contract (keep the use Mix.Task), so we'll just execute them with an array of arguments that has been passed to a custom command shell script with (release_remote_ctl()).

An exception is the migrations: it would need to be outside of a running node (because before running the migrations you'll have to restart the node to get the new code; and the new code may/will crash because it misses the migrations) with release_ctl(). We can also envisage to just run them automatically in Pleroma.Application.start/2.

(note: Mix.Task.run("app.start") should be remplaced with Application.ensure_all_started(:pleroma)).

(depends on !568 (closed) to properly test with a release built by distillery.)