Add new transcribe files microservice

This adds a new "transcribe files" microservice. The primary initial usecase is OCR. Transcription commands are FPR commands, like normalization. OCR should be saved into a SIP in objects/metadata/OCRfiles.

OCR data will be included in a DIP if it exists, and will be copied into an `OCRfiles` directory at the root of the DIP.

Requirements are here: https://www.archivematica.org/wiki/OCR_text_in_DIP

This depends on #22.

~~Note that this isn't 100% ready to merge: I need to unify the post-normalization chains first. Currently transcribeFile is only called in one of them.~~