[Task]: Support having multiple block submissions in parallel

[diox]

Description

As part of implementing automated actions depending policy violations, we should make it possible to have 2 different delayed block submissions co-exist for the same guid + version ids, so that we can have in parallel:

• A BlocklistSubmission that soft-blocks delayed in X days
• A BlocklistSubmission that hard-blocks the same guid+versions, delayed in Y days

Currently, under the hood it mostly already works and mozilla/addons-server#24570 makes use of this, but the admin tool doesn't allow you to create a submission for version ids that already have a pending one. In this ticket we should:

• Make sure we handle conflicting blocklistsubmissions in a predictable way. The main thing to worry about is ensuring we never downgrade a block through a ADDCHANGE action.
• Lift that restriction about multiple blocklist submissions for admins, possibly warning them about the potential conflict

Acceptance Criteria

• item 1

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