Implement PDF processing. Includes:

* Numerous spec improvements
* Refactor the existing processor classes to have more logical names
* Create a way to dispatch to different processors based on mime types
* Fix the carrierwave config
* Remove unused minitests

Todo:

- [ ] Implement the logic that actually parses PDF content
`end` at 10, 7 is not aligned with `it { should validate_attachment_content_type(:upload).allowing(` at 5, 4