Prevent scheduling a new rule with a duplicate alias

[michellemounde]

Fixes #2942

1. Add duplicate alias check at API level to RuleScheduledChangesView

Before insert:

On insert:

[bhearsum]

This is an OK start, but more is needed. Specifically, we should reject duplicate aliases when:

• The alias exists in any row in the rules table
• The alias exists in any row in the rule_scheduled_changes table that meets both the following conditions:
  • complete is False
  • change_type is insert or update

We also need tests for these cases in https://github.com/mozilla-releng/balrog/blob/main/tests/admin/views/test_rules.py

[michellemounde]

Ok @bhearsum will work on the changes. I have a question regarding the failing check. I've looked at the details but I am not sure how to fix the issue so that the check passes.

[bhearsum]

Ok @bhearsum will work on the changes. I have a question regarding the failing check. I've looked at the details but I am not sure how to fix the issue so that the check passes.

That error looks unrelated to your change. It will probably fix itself the next time you push - don't worry about it.

[bhearsum]

This ended up getting fixed in #3010 first.