Ensure we clear stashes when uploading new blocklist filters

[KevinMind]

Fixes: mozilla/addons#15294

Description

• Adding a new BlockListAction enum to define specific upload actions
• Refactoring "upload_filter" to accept a programmable set of "actions" simplifying the underlying logic and making future modifications to how and what we upload easier to control from the cron
• Change business logic in "upload_mlbf_to_remote_settings" to upload either both filters if either filter exceeds the replace threshold, simplifying the cleanup logic for stashes in "upload_filter"

Context

Why add actions. The reason two fold. 1) This is very readable and makes it clear what we expect upload_filter to do. 2) In a future where we want a more granular or sophisticated set of actions to perform, we can add them without greatly interrupting the existing set of code. This will make further improvements simpler and easier to review.

Testing

In a nutshell, use all of these scenarios excepting:

• for any case where we would have uploaded a filter and a stash, now we upload 2 filters.
• when we upload a filter, we expect both filters to be uploaded and for all previous records to be removed from remote settings

You can follow setup instructions here

Note

You won't need to set enable-soft-blocking any more as that switch is removed in this patch

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.

[diox]

That works. Some comments while I play around with it more.