Clean up exercise files

[BNAndras]

@kotp, I'd like to propose some cleanup.

For starters, I'd like to go through the remaining exercises and fill out the empty file stubs.

(defmodule exercise-slug
  (export (function-being-tested 1)))

  ; Please implement the function-being-tested function.

Second, I want to drop the .gitignore files in each exercise since they're largely clutter. We already have a global .gitignore file.

Third, I want to fill out the description for every exercise's src/exercise-slug.app.src file since make test wants this filled out on some LFE versions. https://github.com/exercism/erlang/blob/881e88d00dc42176b046b743f566a40625fbd887/exercises/practice/clock/src/clock.app.src#L2 is an example of how that's filled out on the Erlang track, and we can do something similar like "exercism.org - clock".

Fourth, the rebar config files are marked as invalidators, but I think the rebar lock and app.src files should be as well.

Fifth, the indent size is set to 4 spaces, but I think this should be set to 2 spaces judging from the LFE code.

Anything else I missed or something I shouldn't do from this initial list?

[kotp]

• Populate file stubs
• Remove individual .gitignore files for exercises
• Populate src/exercise-slug.app.src
• Expand "invalidator": [ "rebar.config" ] entry to include rebar.lock and app.src files
• Indentation to two spaces.

The Lisp Style Tips for the Beginner is a pretty clear no-nonsense (and less "by consensus, check your editor") reference, I think.