Switch to company mode

This is a bit messy because it does combine a few things but most of the messyness is isolated into my personal overrides files.

- Removes `auto-complete` in favor of using `company` for completion.
- Removes `clojure-test-mode`, `ac-nrepl`,  (thought this was already on master but guess not). Both are deprecated by newer modes for completion and clojure-mode itself.

Moving to `company` instead of `auto-complete` despite the feature parity not being there (documentation pop-up no longer supported) because it seems to have better behavior than `auto-complete` with `cider`. There is an open issue on `company` about supporting documentation pop-ups. We'll see what happens.