Refactor MLBF in preparation for soft blocking

[KevinMind]

Relates to: mozilla/addons#15014

Description

• Introduce by_block_type queryset on BlockVerssion model to query blocked versions by block type consistently.
• Add BlockType enum to model block type consistently.
• Add MLBF_File to model the additional mlbf files "stash" "notblocked"
• Derive paths in MLBF from storage (with improved path helper) based on MLBF_File input
• Add dump/load/path helpers for handling json data and deriving paths
• Ever so slightly improve the type hints for the MLBF methods.

Context

This PR should introduce zero changes to the public API of MLBF or the functionality but sets the stage for adding additional block types for deriving filters/stashes per type.

The current implementation is really designed for a binary blocked/unlblocked but we will now need 2 binary states and we can reduce some boilerplate and introduce the concepts for doing that in a PR that doesn't actually require changing the API, reducing the size of the PR introducing soft blocking.

Testing

CI passes.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.