Add ability to harden / soften blocks in the admin

[diox]

Fixes: mozilla/addons#15084

Description

See title, this allows admins to harden or soften a block (either partially or completely).

Context

Like with the rest of the block operations, you're never directly modifying Block or BlockVersion for this in the admin, instead you go through a BlocklistSubmission with a specific action. This allows signoff and makes the UI consistent across the various blocklist-related features in the admin.

Testing

There should be harden & soften buttons for a block. Once on the corresponding page, you should be able to select only the hard-blocked versions in that block for the soften action, and the opposite, soft-blocked versions for the harden action. Once you submit, the selected versions should have their block type changed according to the action performed.

The signoff mechanism is reused, untouched. So, if the change requires signoff (average daily users for the add-on is above configured threshold of settings.DUAL_SIGNOFF_AVERAGE_DAILY_USERS_THRESHOLD), the changes are pending approval, which is performed as normal in the same admin (in production it would be a different user, if DEBUG is True we allow the same user)

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]

Screenshots

Slight copy changes on Delete Block page (call the action next to the blocked version "Unblock")

Hardening a complex block

Note that only the soft-blocked version is available to select

Softening a complex block

Opposite as before

Soften / Harden buttons shown on Block page

Both buttons available

Only Softening button available

Only Hardening button available

[eviljeff]

Just nits

[diox]

@eviljeff went ahead and used extended choices in c59d742 ; re-requesting approval because it's a large change (as you said, marginal benefits, but it does address the couple minor points you raised, so I figured it was worth a try and could be useful in the long run)

[eviljeff]