Fix: Allow setting mailrule order from frontend

[shamoon]

Proposed change

Does what it says on the tin

Fixes #2455

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please explain)

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[coveralls]

Pull Request Test Coverage Report for Build 3942442488

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 92.611%

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Totals
Change from base Build 3941897400:	-0.1%
Covered Lines:	5339
Relevant Lines:	5765

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3942442488

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 92.721%

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Files with Coverage Reduction	New Missed Lines	%
documents/signals/handlers.py	1	88.57%

Totals
Change from base Build 3941897400:	-0.02%
Covered Lines:	5439
Relevant Lines:	5866

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💛 - Coveralls

[github-actions]

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